Messages in this thread | | | Date | Thu, 3 Mar 2016 14:46:25 +0000 | From | One Thousand Gnomes <> | Subject | Re: allocate an official device major number for virtio device? |
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On Thu, 3 Mar 2016 03:52:20 +0000 "Ning, Yu" <yu.ning@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
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