linux-kernel - Re: allocate an official device major number for virtio device?

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Date:	Thu, 3 Mar 2016 14:46:25 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	"Ning, Yu" <yu.ning@...el.com>
Cc:	Jin Qian <jinqian@...roid.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Jeff Sharkey" <jsharkey@...gle.com>,
	David Turner <digit@...gle.com>,
	"pprabhu@...gle.com" <pprabhu@...gle.com>
Subject: Re: allocate an official device major number for virtio device?

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