Re: allocate an official device major number for virtio device?

From: One Thousand Gnomes
Date: Thu Mar 03 2016 - 09:46:49 EST


On Thu, 3 Mar 2016 03:52:20 +0000
"Ning, Yu" <yu.ning@xxxxxxxxx> wrote:

> Well, virtio_blk does use dynamic major number allocation, but the allocated block major just happens to fall in the "experimental" range (240-254)...
>
> In more detail:
>
> virtio_blk calls register_blkdev() with major = 0 in init() (drivers/block/virtio_blk.c:872):
>
> major = register_blkdev(0, "virtblk");
>
> This line has been there since day one. And register_blkdev() implements dynamic major allocation pretty straightforwardly:
>
> /* temporary */
> if (major == 0) {
> for (index = ARRAY_SIZE(major_names)-1; index > 0; index--) {
> if (major_names[index] == NULL)
> break;
> }
>
> So it goes from index = 254 to 1 and picks the first unused. Apparently, there's a good chance that the allocated major is between 240-254 (although lower numbers are also possible, theoretically). Indeed, we always get 253 for virtio_blk with the x86_64 Android emulator kernel.
>
> But "dynamic" means we can't rely on checking major == 253 to detect virtio_blk. That's why we are doing a fnmatch() using pattern /sys/devices/*/block/vd* instead. Is that the recommended approach?

That sounds fine to me - if you desperately need to know the assigned
major you can look in /proc/devices

Alan