1 files changed, 114 insertions, 0 deletions
diff --git a/include/linux/kdev_t.h b/include/linux/kdev_t.h
new file mode 100644
index 000000000..a06cdb9ed
--- /dev/null
+++ b/include/linux/kdev_t.h
@@ -0,0 +1,114 @@
+#ifndef _LINUX_KDEV_T_H
+#define _LINUX_KDEV_T_H
+#ifdef __KERNEL__
+/*
+As a preparation for the introduction of larger device numbers,
+we introduce a type kdev_t to hold them. No information about
+this type is known outside of this include file.
+
+Objects of type kdev_t designate a device. Outside of the kernel
+the corresponding things are objects of type dev_t - usually an
+integral type with the device major and minor in the high and low
+bits, respectively. Conversion is done by
+
+extern kdev_t to_kdev_t(int);
+
+It is up to the various file systems to decide how objects of type
+dev_t are stored on disk.
+The only other point of contact between kernel and outside world
+are the system calls stat and mknod, new versions of which will
+eventually have to be used in libc.
+
+[Unfortunately, the floppy control ioctls fail to hide the internal
+kernel structures, and the fd_device field of a struct floppy_drive_struct
+is user-visible. So, it remains a dev_t for the moment, with some ugly
+conversions in floppy.c.]
+
+Inside the kernel, we aim for a kdev_t type that is a pointer
+to a structure with information about the device (like major,
+minor, size, blocksize, sectorsize, name, read-only flag,
+struct file_operations etc.).
+
+However, for the time being we let kdev_t be almost the same as dev_t:
+
+typedef struct { unsigned short major, minor; } kdev_t;
+
+Admissible operations on an object of type kdev_t:
+- passing it along
+- comparing it for equality with another such object
+- storing it in ROOT_DEV, inode->i_dev, inode->i_rdev, sb->s_dev,
+  bh->b_dev, req->rq_dev, de->dc_dev, tty->device
+- using its bit pattern as argument in a hash function
+- finding its major and minor
+- complaining about it
+
+An object of type kdev_t is created only by the function MKDEV(),
+with the single exception of the constant 0 (no device).
+
+Right now the other information mentioned above is usually found
+in static arrays indexed by major or major,minor.
+
+An obstacle to immediately using
+    typedef struct { ... (* lots of information *) } *kdev_t
+is the case of mknod used to create a block device that the
+kernel doesn't know about at present (but first learns about
+when some module is inserted).
+
+aeb - 950811
+*/
+
+/* Since MINOR(dev) is used as index in static arrays,
+   the kernel is not quite ready yet for larger minors.
+   However, everything runs fine with an arbitrary kdev_t type. */
+
+#define MINORBITS	8
+#define MINORMASK	((1U << MINORBITS) - 1)
+
+typedef unsigned short kdev_t;
+
+#define MAJOR(dev)	((unsigned int) ((dev) >> MINORBITS))
+#define MINOR(dev)	((unsigned int) ((dev) & MINORMASK))
+#define HASHDEV(dev)	((unsigned int) (dev))
+#define NODEV		0
+#define MKDEV(ma,mi)	(((ma) << MINORBITS) | (mi))
+#define B_FREE		0xffff		/* yuk */
+
+extern char * kdevname(kdev_t);	/* note: returns pointer to static data! */
+
+/*
+As long as device numbers in the outside world have 16 bits only,
+we use these conversions.
+*/
+
+static inline unsigned int kdev_t_to_nr(kdev_t dev) {
+	return (MAJOR(dev)<<8) | MINOR(dev);
+}
+
+static inline kdev_t to_kdev_t(int dev)
+{
+	int major, minor;
+#if 0
+	major = (dev >> 16);
+	if (!major) {
+		major = (dev >> 8);
+		minor = (dev & 0xff);
+	} else
+		minor = (dev & 0xffff);
+#else
+	major = (dev >> 8);
+	minor = (dev & 0xff);
+#endif
+	return MKDEV(major, minor);
+}
+
+#else /* __KERNEL__ */
+
+/*
+Some programs want their definitions of MAJOR and MINOR and MKDEV
+from the kernel sources. These must be the externally visible ones.
+*/
+#define MAJOR(dev)	((dev)>>8)
+#define MINOR(dev)	((dev) & 0xff)
+#define MKDEV(ma,mi)	((ma)<<8 | (mi))
+#endif /* __KERNEL__ */
+#endif