[RFC] /proc/devices for block devices

[RFC] /proc/devices for block devices

[Christoph Hellwig]

With the bdevname changes now in Linus' tree get_blkdev_list, the
routine that generates the block device output for /proc/devices is the
last consumer of the major_names list in drivers/block/genhd.c.  It's
the last left-over from the old-style per-major blockdevice registration
({,un}register_blkdev) and I don't think it's a good idea to keep this
crap around just to generate a list that can't fully describe the block
dev_t allocations anyway.

We could either

 (a) nuke it completly - if you need to find out the dev_t for your
     block driver with dynamic dev_t allocation you can look at
     /proc/partitions (portable to older kernels) or sysfs instead.
 (b) add some cludges to generate a list from the gendisk structures,
     this would mean every gendisk appears in the list so it would
     be a lot longer an contain slightly different names (with the
     gendisk name appended) - OTOH this is still enough for the
     simple string comparisms in the userspace tools I know that
     use it, like the libdisk volume manager detection code.

Personally I'd strongly prefer a.  Attached is a simple patch that
implements (b), the implementation of (a) is left as an exercise
to the reader.


--- 1.84/drivers/block/genhd.c	Sun May  4 10:38:05 2003
+++ edited/drivers/block/genhd.c	Sat May 10 23:03:05 2003
@@ -19,18 +19,14 @@
 static struct subsystem block_subsys;
 
 /*
- * Can be merged with blk_probe or deleted altogether. Later.
- *
- * Modified under both block_subsys.rwsem and major_names_lock.
+ * Will be deleted altogether. Later.
+ * Modified under block_subsys.rwsem.
  */
 static struct blk_major_name {
 	struct blk_major_name *next;
 	int major;
-	char name[16];
 } *major_names[MAX_PROBE_HASH];
 
-static spinlock_t major_names_lock = SPIN_LOCK_UNLOCKED;
-
 static struct blk_probe {
 	struct blk_probe *next;
 	dev_t dev;
@@ -52,22 +48,15 @@
 	return major_to_index(MAJOR(dev));
 }
 
-/* get block device names in somewhat random order */
 int get_blkdev_list(char *p)
 {
-	struct blk_major_name *n;
-	int i, len;
-
-	len = sprintf(p, "\nBlock devices:\n");
+	int len = sprintf(p, "\nBlock devices:\n");
+	struct gendisk *disk;
 
 	down_read(&block_subsys.rwsem);
-	for (i = 0; i < ARRAY_SIZE(major_names); i++) {
-		for (n = major_names[i]; n; n = n->next)
-			len += sprintf(p+len, "%3d %s\n",
-				       n->major, n->name);
-	}
+	list_for_each_entry(disk, &block_subsys.kset.list, kobj.entry)
+		len += sprintf(p+len, "%3d %s\n", disk->major, disk->disk_name);
 	up_read(&block_subsys.rwsem);
-
 	return len;
 }
 
@@ -75,7 +64,6 @@
 {
 	struct blk_major_name **n, *p;
 	int index, ret = 0;
-	unsigned long flags;
 
 	down_write(&block_subsys.rwsem);
 
@@ -103,12 +91,9 @@
 	}
 
 	p->major = major;
-	strncpy(p->name, name, sizeof(p->name)-1);
-	p->name[sizeof(p->name)-1] = 0;
 	p->next = 0;
 	index = major_to_index(major);
 
-	spin_lock_irqsave(&major_names_lock, flags);
 	for (n = &major_names[index]; *n; n = &(*n)->next) {
 		if ((*n)->major == major)
 			break;
@@ -117,7 +102,6 @@
 		*n = p;
 	else
 		ret = -EBUSY;
-	spin_unlock_irqrestore(&major_names_lock, flags);
 
 	if (ret < 0) {
 		printk("register_blkdev: cannot get major %d for %s\n",
@@ -135,21 +119,18 @@
 	struct blk_major_name **n;
 	struct blk_major_name *p = NULL;
 	int index = major_to_index(major);
-	unsigned long flags;
 	int ret = 0;
 
 	down_write(&block_subsys.rwsem);
-	spin_lock_irqsave(&major_names_lock, flags);
 	for (n = &major_names[index]; *n; n = &(*n)->next)
 		if ((*n)->major == major)
 			break;
-	if (!*n || strcmp((*n)->name, name))
+	if (!*n)
 		ret = -EINVAL;
 	else {
 		p = *n;
 		*n = p->next;
 	}
-	spin_unlock_irqrestore(&major_names_lock, flags);
 	up_write(&block_subsys.rwsem);
 	kfree(p);
 

Re: [RFC] /proc/devices for block devices

[Andries Brouwer]

Christoph Hellwig wrote [on /proc/devices]:

> We could either
> 
>  (a) nuke it completly - if you need to find out the dev_t for your
>      block driver with dynamic dev_t allocation you can look at
>      /proc/partitions (portable to older kernels) or sysfs instead.
>  (b) add some cludges

A grep on a random source tree shows:

A number of programs search /proc/devices for their favourite string,
find the device number and do mknod on it. Thus apmd for "apm_bios"
and IMonLinux for "imon" and hwcrypto for "paep" and "cryptonet".

Apart from for finding out dev_t, /proc/devices is used for
just checking that a certain driver exists in a certain kernel.

There are FAQs that tell people to check for "lp" or "sg"
or "sound" or "pcmcia" in /proc/devices.

The program anaconda checks /proc/devices for "fd" and for "pcmcia".
Various initscripts check for "usb", "sound", "sparcaudio".
SANE checks for "sg". Setserial checks for "ttyS".
Lomount and mount check for "loop".

LVM programs go the other way and use /proc/devices to find out
whether the device with given device number is of type "ide", "sd",
"nbd", "loop" etc.

Andries