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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Mar 03, 2016 at 11:32:00AM -0800, Jin Qian wrote:
> On Thu, Mar 3, 2016 at 9:18 AM, Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> > A: No.
> > Q: Should I include quotations after my reply?
> >
> > http://daringfireball.net/2007/07/on_top
> >
> > On Thu, Mar 03, 2016 at 03:52:20AM +0000, Ning, Yu 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)...
> >
> > That all depends on what else is registered in the system at the moment.
> >
> >> 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.
> >
> > Nor should you, why would you care?
> >
> >> That's why we are doing a fnmatch() using pattern
> >> /sys/devices/*/block/vd* instead.  Is that the recommended approach?
> >
> > Yes, or just look at the device node that is already created in /dev/
> > for you automatially by devtmpfs.  Doesn't that work as expected today?
> >
> > I still don't understand the issue you are having here at all, sorry.
> 
> Maybe Jeff can comment in more detail about the issue and which method
> is preferred?
> 
> Just to summarize, we have a few ways to detect the device
> 
> 1. match /sys/devices/...

/sys/dev/block/ you mean.

> 2. match /dev/...

Yes.

> 3. use uevent rule to detect virtio device creation

Yes.

> 4. check a fixed major#

Don't do that, it's not going to work as you don't have an assigned
major number.

But again, what do you mean by "detect the device"?  What do you want to
do here, just open/mount it?  Why don't the existing tools just handle
this automatically for you?

What do you want to do when you "detect the device"?  What needs this?

thanks,

greg k-h