path: root/Documentation/s390/cds.txt

Linux/390

Common Device Support (CDS)
Device Driver I/O Support Routines

Author : Ingo Adlung

Copyright, IBM Corp. 1999

Introduction

This document describes the common device support routines for Linux/390.
Different than other hardware architectures, ESA/390 hasdefined a unified
I/O access method. This gives relief to the device drivers as they don't
have to deal with different bus types, polling versus interrupt
processing, shared versus non-shared interrupt processing, DMA versus port
I/O (PIO), and other hardware features more. However, this implies that
either every single device driver needs to implement the hardware I/O
attachment functionality itself, or the operating system provides for a
unified method to access the hardware, providing all the functionality that
every single device driver would have to provide itself.

The document does not intend to explain the ESA/390 hardware architecture in
every detail.This information can be obtained from the ESA/390 Principles of
Operation manual (IBM Form. No. SA22-7201).

In order to build common device support for ESA/390 I/O interfaces, a
functional layer was introduced that provides generic I/O access methods to
the hardware. The following figure shows the usage of the common device support
of Linux/390 using a TbCP/IP driven device access an example. Similar figures
could be drawn for other access methods, e.g. file system access to disk
devices.

The common device support layer shown above comprises the I/O support routines
defined below. Some of them implement common Linux device driver interfaces,
while some of them are ESA/390 platform specific.

get_dev_info_by_irq() / get_dev_info_by_devno()	
   allow a device driver to determine the devices attached (visible) to the
   system and their current status.

get_irq_by_devno() / get_devno_by_irq()	
   get irq (subchannel) from device number and vice versa.

read_dev_chars()	
   read device characteristics

request_irq()	
   obtain ownership for a specific device.

free_irq()	
   release ownership for a specific device.

disable_irq()	
   disable a device from presenting interrupts.

enable_irq()	
   enable a device, allowing for I/O interrupts.

do_IO()	
   initiate an I/O request.

halt_IO()	
   terminate the current I/O request processed on the device.

do_IRQ()	
   generic interrupt routine. This function is called by the interrupt entry
   routine whenever an I/O interrupt is presented to the system. The do_IRQ()
   routine determines the interrupt status and calls the device specific
   interrupt handler according to the rules (flags) defined during I/O request
   initiation with do_IO().

The next chapters describe the functions, other than do_IRQ() in more details.
The do_IRQ() interface is not described, as it is called from the Linux/390
first level interrupt handler only and does not comprise a device driver
callable interface. Instead, the functional description of do_IO() also
describes the input to the device specific interrupt handler.


Common Device Support (CDS) for Linux/390 Device Drivers

General Information

The following chapters describe the I/O related interface routines the
Linux/390 common device support (CDS) provides to allow for device specific
driver implementations on the IBM ESA/390 hardware platform. Those interfaces
intend to provide the functionality required by every device driver
implementaion to allow to drive a specific hardware device on the ESA/390
platform. Some of the interface routines are specific to Linux/390 and some
of them can be found on other Linux platforms implementations too.
Miscellaneous function prototypes, data declarations, and macro definitions
can be found in the architecture specific C header file
linux/arch/s390/kernel/irq.h.

Overview of CDS interface concepts

Different to other hardware platforms, the ESA/390 architecture doesn't define
interrupt lines managed by a specific interrupt controller and bus systems
that may or may not allow for shared interrupts, DMA processing, etc.. Instead,
the ESA/390 architecture has implemented a so called channel subsystem, that
provides a unified view of the devices physically attached to the systems.
Though the ESA/390 hardware platform knows about a huge variety of different
peripheral attachments like disk devices (aka. DASDs), tapes, communication
controllers, etc. they can all by accessed by a well defined access method and
they are presenting I/O completion a unified way : I/O interruptions. Every
single device is uniquely identified to the system by a so called subchannel,
where the ESA/390 architecture allows for 64k devices be attached.

Linux, however was first built on the Intel PC architecture, with its two
cascaded 8259 programmable interrupt controllers (PICs), that allow for a
maximum of 15 different interrupt lines. All devices attached to such a system
share those 15 interrupt levels. Devices attached to the ISA bus system must
not share interrupt levels (aka. IRQs), as the ISA bus bases on edge triggered
interrupts. MCA, EISA, PCI and other bus systems base on level triggered
interrupts, and therewith allow for shared IRQs. However, if multiple devices
present their hardware status by the same (shared) IRQ, the operating system
has to call every single device driver registered on this IRQ in order to
determine the device driver owning the device that raised the interrupt.

In order to not introduce a new I/O concept to the common Linux code,
Linux/390 preserves the IRQ concept and semantically maps the ESA/390
subchannels to Linux as IRQs. This allows Linux/390 to support up to 64k
different IRQs, uniquely representig a single device each.

During its startup the Linux/390 system checks for peripheral devices. Each
of those devices is uniquely defined by a so called subchannel by the ESA/390
channel subsystem. While the subchannel numbers are system generated, each
subchannel also takes a user defined attribute, the so called device number.
Both, subchannel number and device number can not exceed 65535. The
init_IRQ() routine gathers the information about control unit type and device
types that imply specific I/O commands (channel command words - CCWs) in
order to operate the device. Device drivers can retrieve this set of hardware
information during their initialization step to recognize the devices they
support using get_dev_info_by_irq() or get_dev_info_by_devno() respectively.
This methods implies that Linux/390 doesn't require to probe for free (not
armed) interrupt request lines (IRQs) to drive its devices with. Where
applicable, the device drivers can use the read_dev_chars() to retrieve device
characteristics. This can be done without having to request device ownership
previously.

When a device driver has recognized a device it wants to claim ownership for,
it calls request_irq() with the device's subchannel id serving as pseudo irq
line. One of the required parameters it has to specify is dev_id, defining a
device status block, the CDS layer will use to notify the device driver's
interrupt handler about interrupt information observed. It depends on the
device driver to properly handle those interrupts.

In order to allow for easy I/O initiation the CDS layer provides a do_IO()
interface that takes a device specific channel program (one or more CCWs) as
input sets up the required architecture specific control blocks and initiates
an I/O request on behalf of the device driver. The do_IO() routine allows for
different I/O methods, synchronous and asynchronous, and allows to specify
whether it expects the CDS layer to notify the device driver for every
interrupt it observes, or with final status only. It also provides a scheme
to allow for overlapped I/O processing. See do_IO() for more details. A device
driver must never issue ESA/390 I/O commands itself, but must use the
Linux/390 CDS interfaces instead.

For long running I/O request to be canceled, the CDS layer provides the
halt_IO() function. Some devices require to initially issue a HALT SUBCHANNEL
(HSCH) command without having pending I/O requests. This function is also
covered by halt_IO().

When done with a device, the device driver calls free_irq() to release its
ownership for the device. During free_irq() processing the CDS layer also
disables the device from presenting further interrupts - the device driver
doesn't need to assure it. The device will be reenabled for interrupts with
the next call to request_irq().



get_dev_info_by_irq() / get_dev_info_by_devno() - Retrieve Device Information

During system startup - init_IRQ() processing - the generic I/O device support
checks for the devices available. For all devices found it collects the
SenseID information. For those devices supporting the command it also obtains
extended SenseID information.

int get_dev_info_by_irq( int         irq,
                         dev_info_t *devinfo);

int get_dev_info_by_devno( unsigned int  irq,
                           dev_info_t   *devinfo);

irq     - defines the subchannel, status information is to be
          returned for.
devno   - device number.
devinfo - pointer to a user buffer of type dev_info_t that should
          be filled with device specific information.

typedef struct {
     unsigned int devno;     /* device number */
     unsigned int status;    /* device status */
     senseid_t    sid_data;  /* senseID data  */
} dev_info_t;

devno     - device number as configured in the IOCDS.
status    - device status
sid_data  - data obtained by a SenseID call

Possible status values are :

DEVSTAT_NOT_OPER - device was found not-operational. In this case
                   the caller should disregard the sid_data
                   buffer content.

//
// SenseID response buffer layout
//
typedef struct {
  /* common part */
      unsigned char  reserved;     /* always 0x'FF' */
      unsigned short cu_type;      /* control unit type */
      unsigned char  cu_model;     /* control unit model */
      unsigned short dev_type;     /* device type */
      unsigned char  dev_model;    /* device model */
      unsigned char  unused;       /* padding byte */
  /* extended part */
      ciw_t    ciw[62];            /* variable # of CIWs */
} senseid_t;

The ESA/390 I/O architecture defines certain device specific I/O functions.
The device returns the device specific command code together with the SenseID
data in so called Command Information Words (CIW) :

typedef struct _ciw {
   unsigned int et       :  2;    // entry type
   unsigned int reserved :  2;    // reserved
   unsigned int ct       :  4;    // command type
   unsigned int cmd      :  8;    // command
   unsigned int count    : 16;    // count
} ciw_t;

Possible CIW entry types are :

#define CIW_TYPE_RDC    0x0;      // read configuration data
#define CIW_TYPE_SII    0x1;      // set interface identifier
#define CIW_TYPE_RNI    0x2;      // read node identifier

The device driver may use these commands as appropriate.

The get_dev_info_by_irq() / get_dev_info_by_devno() functions return:

      0 - sucessful completion
-ENODEV - irq or devno don't specify a known subchannel or device
          number.
-EINVAL - invalid devinfo value.

Usage Notes :

In order to scan for known devices a device driver should scan all irqs by
calling     get_dev_info() until it returns -ENODEV as there aren't any more
available devices.

If a device driver wants to request ownership for a specific device it must
call request_irq() prior to be able to issue any I/O request for it, including
above mentioned   device dependent commands.

Please see the "ESA/390 Common I/O-Commandss and Self Description" manual,
with IBM form number SA22-7204 for more details on how to read the Sense-ID
output, CIWs and device independent commands.



get_irq_by_devno() / get_devno_by_irq() - Convert device identifiers

While some device drivers act on the irq (subchannel) only, others take user
defined device configurations on device number base, according to the device
numbers configured in the IOCDS. The following routines serve the purpose to
convert irq values into device numbers and vice versa.

int get_irq_by_devno( unsigned int devno );

unsigned int get_devno_by_irq( int irq );

The functions return :

the requested irq/devno values
-1 if the requested conversion can't be accomplished.

This could either be caused by irq/devno be outside the valid range
( value > 0xffff or value < 0 ) or not identifying a known device.


read_dev_chars() - Read Device Characteristics

This routine returns the characteristics for the device specified.

The function is meant to be called without an irq handler be in place.
However, the irq for the requested device must not be locked or this will
cause a deadlock situation ! Further, the driver must assure that nobody
else has claimed ownership for the requested irq yet or the owning device
driver's internal accounting may be affected.

In case of a registered interrupt handler, the interrupt handler must be
able to properly react on interrupts related to the read_dev_chars() I/O
commands. While the request is procesed synchronously, the device interrupt
handler is called for final ending status. In case of error situations the
interrupt handler may recover appropriately. The device irq handler can
recognize the corresponding interrupts by the interruption parameter be
0x00524443. If using the function with an existing device interrupt handler
in place, the irq must be locked prior to call read_dev_chars().

The function may be called enabled or disabled.

int read_dev_chars( int irq, void **buffer, int length );

irq    - specifies the subchannel the device characteristic
         retrieval is requested for
buffer - pointer to a buffer pointer. The buffer pointer itself
         may be NULL to have the function allocate a buffer or
         must contain a valid buffer area.
length - length of the buffer provided or to be allocated.

The read_dev_chars() function returns :

      0 - successful completion
-ENODEV - irq doesn't specify a valid subchannel number
-EINVAL - an invalid parameter was detected
-EBUSY  - an irrecoverable I/O error occured or the device is not
          operational.

Usage Notes :

The function can be used in two ways :

If the caller doesn't provide a data buffer, read_dev_chars() allocates a
data buffer and provides the device characteristics together. It's the
caller's responsability to release the kernel memory if not longer needed.
This behaviour is triggered by specifying a NULL buffer area (*buffer == NULL).

Alternatively, if the user specifies a buffer area himself, nothing is
allocated.

In either case the caller must provide the data area length - for the buffer
he specifies, or the buffer he wants to be allocated.


request_irq() - Request Device Ownership

As previously discussed a device driver will scan for the devices its supports
by calling get_dev_info(). Once it has found a device it will call
request_irq() to request ownership for it. This call causes the subchannel to
be enabled for interrupts if it was found operational.

int request_irq( unsigned int   irq,
                 int          (*handler)( int,
                                          void *,
                                          struct pt_regs *),
                 unsigned long  irqflags,
                 const char    *devname,
                 void          *dev_id);

irq      : specifies the subchannel the ownership is requested for
handler  : specifies the device driver's interrupt handler to be
           called for interrupt processing
irqflags : IRQ flags, must be 0 (zero) or SA_SAMPLE_RANDOM
devname  : device name
dev_id   : required pointer to a device specific buffer of type
           devstat_t

typedef struct {
     unsigned int  devno;   /* device number from irb */
     unsigned int  intparm; /* interrupt parameter */
     unsigned char cstat;   /* channel status - accumulated */
     unsigned char dstat;   /* device status - accumulated */
     unsigned char lpum;    /* last path used mask from irb */
     unsigned char unused;  /* not used - reserved */
     unsigned int  flag;    /* flag : see below */
     unsigned long cpa;     /* CCW addr from irb at prim. status */
     unsigned int  rescnt;  /* count from irb at primary status */
     unsigned int  scnt;    /* sense count, if available */
     union {
        irb_t   irb;        /* interruption response block */
        sense_t sense;      /* sense information */
        } ii;               /* interrupt information */
  } devstat_t;

During request_irq() processing, the devstat_t layout does not matter as it
won't be used during request_irq() processing. See do_IO() for a functional
description of its usage.

The request_irq() function returns :

      0 - successful completion
-EINVAL - an invalid parameter was detected
-EBUSY  - device (subchannel) already owned
-ENODEV - the device is not-operational
-ENOMEM - not enough kernel memory to process request

Usage Notes :

While Linux for Intel defines dev_id as a unique identifier for shared
interrupt lines it has a totally different purpose on Linux/390. Here it
serves as a shared interrupt status area between the generic device support
layer, and the device specific driver. The value passed to request_irq()
must therefore point to a valid devstat_t type buffer area the device driver
must preserve for later usage. I.e. it must not be released prior to a call
to free_irq()

The only value parameter irqflags supports is SA_SAMPLE_RANDOM if appropriate.
The Linux/390 kernel does neither know about "fast" interrupt handlers, nor
does it allow for interrupt sharing. Remember, the term interrupt level (irq),
device, and subchannel are used interchangeably in Linux/390.

If request_irq() was called in enabled state, or if multiple CPUs are present,
the device may present an interrupt to the specified handler prior to
request_irq() return to the caller  already ! This includes the possibility
of unsolicited interrupts or a pending interrupt status from an earlier
solicited I/O request. The device driver must be able to handle this situation
properly or the device may become unoperational otherwise !

Although the interrupt handler is defined to be called with a pointer to a
struct pt_regs buffer area, this is not implemented by the Linux/390 generic
I/O device driver support layer. The device driver's interrupt handler must
therefore not rely on this parameter on function entry.


free_irq() - Release Device Ownership

A device driver may call free_irq() to release ownership of a previously
aquired device.

void free_irq( unsigned int  irq,
               void         *dev_id);

irq      : specifies the subchannel the ownership is requested for
dev_id   : required pointer to a device specific buffer of type
           devstat_t. This must be the same as the one specified
           during a previous call to request_irq().

Usage Notes :

Unfortunately the free_irq() is defined not to return error codes. I.e. if
called with wrong  parameters a device may still be operational although there
is no device driver available to handle its interrupts. Further, during
free_irq() processing we may possibly find pending interrupt conditions. As
those need to be processed, we have to delay free_irq() returning until a
clean device status is found by synchronously handling them.

The call to free_irq() will also cause the device (subchannel) be disabled for
interrupts. The device driver must not release any data areas required for
interrupt processing prior to free_irq() return to the caller as interrupts
can occur prior to free_irq() returning. This is also true when called in
disabled state if either multiple CPUs are presents or a pending interrupt
status was found during free_irq() processing.


disable_irq() - Disable Interrupts for a given Device

This function may be called at any time to disable interrupt processing for
the specified irq. However, as Linux/390 maps irqs to the device (subchannel)
one-to-one, this may require more extensive I/O processing than anticipated,
especially if an interrupt status is found pending on the subchannel that
requires synchronous error processing.

int disable_irq( unsigned int irq );

irq : specifies the subchannel to be disabled

The disable-irq() routine may return :

      0 - successful completion
-EBUSY  - device (subchannel) is currently processing an I/O
          request
-ENODEV - the device is not-operational or irq doesn't specify a
          valid subchannel

Usage Notes :

Unlike the Intel based hardware architecture the ESA/390 architecture does
not have a programmable interrupt controller (PIC) where a specific interrupt
line can be disabled. Instead the subchannel logically representing the device
in the channel subsystem must be disabled for interrupts. However, if there
are still inetrrupt conditions pending they must   be processed first in order
to allow for proper processing after reenabling the device at a later time.
This may lead to delayed disable processing.

As described above the disable processing may require extensive processing.
Therefore    disabling and re-enabling the device using disable_irq() /
enable_irq() should be avoided and is not suitable for high frequency
operations.

Linux for Intel defines this function

void disable_irq( int irq);

This is suitable for the Intel PC architecture as this only causes to mask
the requested irq line in the PIC which is not applicable for the ESA/390
architecture. Therefore we allow   for returning error codes.


enable_irq() - Enable Interrupts for a given Device

This function is used to enable a previously disabled device (subchannel).
See disable_irq() for more details.

int enable_irq( unsigned int irq );

irq : specifies the subchannel to be enabled

The enable-irq() routine may return :

      0 - successful completion
-EBUSY  - device (subchannel) is currently processing an I/O
          request. This implies the device is already in enabled
          state
-ENODEV - the device is not-operational or irq doesn't specify a
          valid subchannel



do_IO() - Initiate I/O Request

The do_IO() routines is the I/O request front-end processor. All device driver
I/O requests must be issued using this routine. A device driver must not issue
ESA/390 I/O commands itself. Instead the do_IO() routine provides all
interfaces required to drive arbitrary devices.

This description also covers the status information passed to the device
driver's interrupt handler as this is related to the rules (flags) defined
with the associated I/O request when calling do_IO().

int do_IO( int            irq,
           ccw1_t        *cpa,
           unsigned long  intparm,
           unsigned int   lpm,
           unsigned long  flag);

irq     : irq (subchannel) the I/O request is destined for
cpa     : logical start address of channel program
intparm : user specific interrupt information; will be presented
          back to the device driver's interrupt handler. Allows a
          device driver to associate the interrupt with a
          particular I/O request.
lpm     : defines the channel path to be used for a specific I/O
          request. Valid with flag value DOIO_VALID_LPM only.
flag    : defines the action to e parformed for I/O processing

Possible flag values are :

DOIO_EARLY_NOTIFICATION  - allow for early interupt notification
DOIO_VALID_LPM           - LPM input parameter is valid (see usage
                           notes below for details)
DOIO_WAIT_FOR_INTERRUPT  - wait synchronously for final status
DOIO_REPORT_ALL          - report all interrupt conditions

The cpa parameter points to the first format 1 CCW of a channel program :

typedef struct {
   char            cmd_code; /* command code */
   char            flags;    /* flags, like IDA adressing, etc. */
   unsigned short  count;    /* byte count */
   void           *cda;      /* data address */
} ccw1_t __attribute__ ((aligned(8)));

with the following CCW flags values defined :

CCW_FLAG_DC        - data chaining
CCW_FLAG_CC        - command chaining
CCW_FLAG_SLI       - suppress incorrct length
CCW_FLAG_SKIP      - skip
CCW_FLAG_PCI       - PCI
CCW_FLAG_IDA       - indirect addressing
CCW_FLAG_SUSPEND   - suspend



The do_IO() function returns :

      0 - successful completion or request successfuly initiated
-EBUSY  - the do_io() function was caled out of sequence. The
          device is currently processing a previous I/O request
-ENODEV - irq doesn't specify a valid subchannel, the device is
          not operational (check dev_id.flags) or the irq is not
          owned.
-EINVAL - both, DOIO_EARLY_NOTIFICATION and DOIO_REORT_ALL flags
          have been specified. The usage of those flags is mutual
          exclusive.

When the I/O request completes, the CDS first level interrupt handler will
setup the dev_id buffer of type devstat_t defined during request_irq()
processing. See request_irq() for the devstat_t data layout. The
dev_id->intparm field in the device status area will contain the value the
device driver has associated with a particular I/O request. If a pending
device status was recognized dev_id->intparm will be set to 0 (zero). This
may happen during I/O initiation or delayed by an alert status notification.
In any case this status is not related to the current (last) I/O request. In
case of a delayed status notification no special interrupt will be presented
to indicate I/O completion as the I/O request was never started, even though
do_IO() returned with successful completion.

Possible dev_id->flag values are :

DEVSTAT_FLAG_SENSE_AVAIL - sense data is available
DEVSTAT_NOT_OPER         - device is not-operational
DEVSTAT_START_FUNCTION   - interrupt is presented as result of a
                           call to do_IO()
DEVSTAT_HALT_FUNCTION    - interrupt is presented as result of a
                           call to halt_IO()
DEVSTAT_STATUS_PENDING   - a pending status was found. The I/O
                           resquest (if any) was not initiated.
                           This status might have been presented
                           delayed, after do_IO() or halt_IO() have
                           successfully be started previously.
DEVSTAT_FINAL_STATUS     - This is a final interrupt status for the
                           I/O requst identified by intparm.

If device status DEVSTAT_FLAG_SENSE_AVAIL is indicated in field dev_id->flag,
field dev_id->scnt describes the numer of device specific sense bytes
available in the sense area dev_id->ii.sense. No device sensing by the device
driver itself is required.

typedef struct {
   unsigned char res[32];       /* reserved   */
   unsigned char data[32];      /* sense data */
} sense_t;

The device interrupt handler can use the following definitions to investigate
the primary unit check source coded in sense byte 0 :

SNS0_CMD_REJECT         0x80
SNS0_INTERVENTION_REQ   0x40
SNS0_BUS_OUT_CHECK      0x20
SNS0_EQUIPMENT_CHECK    0x10
SNS0_DATA_CHECK         0x08
SNS0_OVERRUN            0x04

Depending on the device status, multiple of those values may be set together.
Please refer to the device specific documentation for details.

The devi_id->cstat field provides the (accumulated) subchannel status :

SCHN_STAT_PCI            - program controlled interrupt
SCHN_STAT_INCORR_LEN     - incorrect length
SCHN_STAT_PROG_CHECK     - program check
SCHN_STAT_PROT_CHECK     - protection check
SCHN_STAT_CHN_DATA_CHK   - channel data check
SCHN_STAT_CHN_CTRL_CHK   - channel control check
SCHN_STAT_INTF_CTRL_CHK  - interface control check
SCHN_STAT_CHAIN_CHECK    - chaining check

The dev_id->dstat field provides the (accumulated) device status :

DEV_STAT_ATTENTION   - attention
DEV_STAT_STAT_MOD    - status modifier
DEV_STAT_CU_END      - control unit end
DEV_STAT_BUSY        - busy
DEV_STAT_CHN_END     - channel end
DEV_STAT_DEV_END     - device end
DEV_STAT_UNIT_CHECK  - unit check
DEV_STAT_UNIT_EXCEP  - unit exception

Please see the ESA/390 Principles of Operation manual for details on the
individual flag meanings.

In rare error situations the device driver may require access to the original
hardware interrupt data beyond the scope of above mentioned infromation. For
those situations the Linux/390 common device support provides the interrupt
response block (IRB) as part of the device status block in dev_id->ii.irb.

Usage Notes :

Prior to call do_IO() the device driver must

assure disabled state, i.e. the I/O mask value in the PSW must be disabled.
This can be accomplished by calling __save_flags( flags). The current PSW
flags are preserved and can be restored by __restore_flags( flags) at a
later time.

If the device driver violates this rule while running in a uni-processor
environment an interrupt might be presented prior to the do_IO() routine
returning to the device driver main path. In this case we will end in a
deadlock situation as the interrupt handler will try to obtain the irq
lock the device driver still owns (see below) !

the driver must assure to hold the device specific lock. This can be
accomplished by

(i)  s390irq_spin_lock( irq), or
(ii) s390irq_spin_lock_irqsave(irq, flags)

Option (i) should be used if the calling routine is running disabled for
I/O interrupts (see above) already. Option (ii) obtains the device gate und
puts the CPU into I/O disabled state by preserving the current PSW flags.
See the descriptions of s390irq_spin_lock() or s390irq_spin_lock_irqsave()
for more details.

The device driver is allowed to issue the next do_IO() call from within its
interrupt handler already. It is not required to schedule a bottom-half,
unless an non deterministicly long running error recovery procedure or
similar needs to be scheduled. During I/O processing the Linux/390 generic
I/O device driver support has already obtained the IRQ lock, i.e. the handler
must not try to obtain it again when calling do_IO() or we end in a deadlock
situation ! Anyway, the device driver's interrupt handler must only call
do_IO() if the handler itself can be entered recursively if do_IO() e.g. finds
a status pending and needs to all the interrupt handler itself.

Device drivers shouldn't heavily rely on DOIO_WAIT_FOR_INTERRUPT synchronous
I/O request processing. All I/O devices, but the console device are driven
using a single shared interrupt subclass (ISC). For sync. processing the
device is temporarily mapped to a special ISC while the calling CPU waits for
I/O completion. As this special ISC is gated, all sync. requests in a SMP
environment are serialized which may cause other CPUs to spin. This service
is therewith primarily meant to be used during device driver initialization
for ease of device setup.

The lpm input parameter might be used for multipath devices shared among
multiple systems as the Linux/390 CDS isn't grouping channel paths. Therefore
its use might be required if multiple access paths to a device are available
and the device was reserved by means of a reserve device command (for devices
supporting this technique). When issuing this command the device driver needs
needs to extract the dev_id->lpum value and restrict all subsequent channel
programs to this channel path until the device is released by a device release
command. Otherwise a deadlock may occur.

If a device driver relies on an I/O request to be completed prior to start the
next it can reduce I/O processing overhead by chaining a NoOp I/O command
CCW_CMD_NOOP to the end of the submitted CCW chain. This will force Channel-End
and Device-End status to be presented together, with a single interrupt.
However, this should be used with care as it implies the channel will remain
busy, not being able to process I/O requests for other devices on the same
channel. Therefore e.g. read commands should never use this technique, as the
result will be presented by a single interrupt anyway.

In order to minimize I/O overhead, a device driver should use the
DOIO_REPORT_ALL  only if the device can report intermediate interrupt
information prior to device-end the device driver urgently relies on. In this
case all I/O interruptions are presented to the device driver until final
status is recognized.

If a device is able to recover from asynchronosly presented I/O errors, it can
perform overlapping I/O using the DOIO_EARLY_NOTIFICATION flag. While some
devices always report channel-end and device-end together, with a single
interrupt, others present primary status (channel-end) when the channel is
ready for the next I/O request and secondary status (device-end) when the data
transmission has been completed at the device.

Above flag allows to exploit this feature, e.g. for communication devices that
can handle lost data on the network to allow for enhanced I/O processing.

Unless the channel subsystem at any time presents a secondary status interrupt,
exploiting this feature will cause only primary status interrups to be
presented to the device driver while overlapping I/O is performed. When a
secondary status without error (alert status) is presented, this indicates
successful completion for all overlapping do_IO() requests that have been
issued since the last secondary (final) status.

During interrupt processing the device specific interrupt handler should avoid
basing its processing decisions on the interruption response block (IRB) that
is part of the dev_id buffer area. The IRB area represents the interruption
parameters from the last interrupt received. Unless the device driver has
specified DOIO_REPORT_ALL or is called with a pending status
(DEVSTAT_STATUS_PENDING), the IRB information may or may not show the complete
interruption status, but the last interrupt only. Therefore the device driver
should usually base its processing decisions on the values of dev_id->cstat
and dev_id->dstat that represent the accumulated subchannel and device status
information gathered since do_IO() request initiation.


halt_IO() - Halt I/O Request Processing

Sometimes a device driver might need a possibility to stop the processing of
a long-running channel program or the device might require to initially issue
a halt subchannel (HSCH) I/O command. For those purposes the halt_IO() command
is provided.

int halt_IO( int          irq,     /* subchannel number */
             int          intparm, /* dummy intparm */
             unsigned int flag);   /* operation mode */

irq     : irq (subchannel) the halt operation is requested for
intparm : interruption parameter; value is only used if no I/O
          is outstanding, otherwise the intparm associated with
          the I/O request is returned
flag    : 0 (zero) or DOIO_WAIT_FOR_INTERRUPT

The halt_IO() function returns :

      0 - successful completion or request successfuly initiated
-EBUSY  - the device is currently performing a sysnchonous I/O
          operation : do_IO() with flag DOIO_WAIT_FOR_INTERRUPT
          or an error was encountered and the device is currently
          be sensed
-ENODEV - the irq specified doesn't specify a valid subchannel, the
          device is not operational (check dev_id.flags) or the irq
          is not owned.

Usage Notes :

A device driver may write a never-ending channel program by writing a channel
program that at its end loops back to its beginning by means of a transfer in
channel (TIC)   command (CCW_CMD_TIC). Usually this is performed by network
device drivers by setting the PCI CCW flag (CCW_FLAG_PCI). Once this CCW is
executed a program controlled interrupt (PCI) is generated. The device driver
can then perform an appropriate action. Prior to interrupt of an outstanding
read to a network device (with or   without PCI flag) a halt_IO() is required
to end the pending operation.

We don't allow to stop sync. I/O requests by means of a halt_IO() call. The
function will return -EBUSY instead.


Miscellaneous Support Routines

This chapter describes various routines to be used in a Linux/390 device
driver programming environment.

s390irq_spin_lock() / s390irq_spin_unlock()

Those two macro definitions are required to obtain the device specific IRQ
lock. The lock needs to be obtained if the device driver intends to call
do_IO() or halt_IO() from anywhere but the device interrupt handler (where
the lock is already owned). Those routines must only be used if running
disabled for interrupts already. Otherwise use s390irq_spin_lock_irqsave()
and the corresponding unlock routine instead (see below).

s390irq_spin_lock( int irq);
s390irq_spin_unlock( int irq);


s390irq_spin_lock_irqsave() / s390_irq_spin_unlock_irqrestore()

Those two macro definitions are required to obtain the device specific IRQ
lock. The lock needs to be obtained if the device driver intends to call
do_IO() or halt_IO() from anywhere but the device interrupt handler (where
the lock is already owned). Those routines should only be used if running
enabled for interrupts. If running disabled already, the driver should use
s390irq_spin_lock() and the corresponding unlock routine instead (see above).

s390irq_spin_lock_irqsave( int irq, unsigned long flags);
s390irq_spin_unlock_irqrestore( int irq, unsigned long flags);




Special Console Interface Routines

This chapter describes the special interface routines required for system
console processing. Though they are an extension to the Linux/390 device
driver interface concept, they base on the same principles. It was necessary
to build those extensions to assure a deterministic behaviour in critical
situations e.g. printk() messages by other device drivers running disabled
for interrupts during I/O interrupt handling or in case of a panic() message
being raised.

set_cons_dev - Set Console Device

This routine allows to specify the system console device. This is necessary
as the console isn't driven by the same ESA/390 interrupt subclass as are
other devices, but it is assigned ist own interrupt subclass. Only one device
can act as system console. See wait_cons_dev() for details.

int set_cons_dev( int irq);

irq : subchannel identifying the system console device

The set_cons_dev() function returns

      0 - successful completion
-EIO    - an unhandled interrupt condition is pending for the
          specified subchannel (irq) - status pending
-ENODEV - irq doesn't specify a valid subchannel or the devive is
          not operational
-EBUSY  - the console device is already defined

reset_cons_dev - Reset Console Device

This routine allows for resetting the console device specification. See
set_cons_dev() for details.

int reset_cons_dev( int irq);

irq : subchannel identifying the system console device

The reset_cons_dev() function returns

      0 - successful completion
-EIO    - an unhandled interrupt condition is pending for the
          specified subchannel (irq) - status pending
-ENODEV - irq doesn't specify a valid subchannel or the devive is
          not operational

wait_cons_dev - Synchronously Wait for Console Processing

The wait_cons_dev() routine is used by the console device driver when its
buffer pool for intermediate request queuing is exhausted and a new output
request is received. In this case the console driver uses the wait_cons_dev()
routine to synchronously wait until enough buffer space is gained to enqueue
the current request. Any pending interrupt condition for the console device
found during wait_cons_dev() processing causes its interrupt handler to be
called.

int wait_cons_dev( int irq);

irq : subchannel identifying the system console device

The wait_cons_dev() function  returns :

      0 - successful completion
-EINVAL - the irq specified doesn't match the irq configured for
          the console device by set_cons_dev()

Usage Notes :

The function should be used carefully. Especially in a SMP environment the
wait_cons_dev() processing requires that all but the special console ISC are
disabled. In a SMP system this requires the other CPUs to be signaled to
disable/enable those ISCs.