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?
Thanks,
Yu
> -----Original Message-----
> From: Jin Qian [mailto:jinq...@android.com]
> Sent: Thursday, March 3, 2016 9:48
> To: Greg Kroah-Hartman <gre...@linuxfoundation.org>
> Cc: linux-kernel@vger.kernel.org; Jeff Sharkey <jshar...@google.com>; David
> Turner <di...@google.com>; ppra...@google.com; Ning, Yu
> <yu.n...@intel.com>
> Subject: Re: allocate an official device major number for virtio device?
>
> Do you mean detecting device name string as in /dev/...?
>
> Just checked latest virtio_blk code, it's dynamic but not using anything
> specific
> to experimental range. I guess we're fine here but Yu can confirm.
>
> Thanks,
> jin
>
> On Wed, Mar 2, 2016 at 5:25 PM, Greg Kroah-Hartman
> <gre...@linuxfoundation.org> wrote:
> > On Wed, Mar 02, 2016 at 05:08:00PM -0800, Jin Qian wrote:
> >> Hi -
> >>
> >> Can we allocate an official device major number for virtio devices?
> >> Currently it's using 240-254 (LOCAL/EXPERIMENTAL USE). The reason we
> >> ask for this is because userspace will need to treat virtio block
> >> devices differently and need a way to detect such device. For
> >> example, it checks major number to detect scsi and mmc device.
> >>
> >> https://android-review.googlesource.com/#/c/195240
> >>
> >> With dynamic major numbers 240-254, we might treat other devices as
> >> virtio block device incorrectly.
> >
> > You shouldn't treat them incorrectly, devtmpfs handles this for you
> > automatically, so I'd recommend using the dynamic majors please.
> >
> > And what in-kernel code is using the "experimental" range today? Do
> > you have a pointer to that? We should fix that now...
> >
> > thanks,
> >
> > greg k-h
