On Thu, 3 Mar 2016 03:52:20 +0000
"Ning, Yu" <yu.n...@intel.com> 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
