Re: [PATCH] usb: xhci: fix maximum event ring segments calculation

[Pierre-David Belanger <pierredavidbelanger@gmail.com>]

I ran into this independently on x86_64 under QEMU and had a regression
report half written when I found your patch. Same one line, same reasoning.

Confirmed against vanilla mainline 08dbfad3f504, with v7.2 built from the same
config for comparison:

  v7.2 (8d3ae59288f1)           USB keyboard enumerates
  08dbfad3f504                  xhci_hcd does not probe
  08dbfad3f504 + this patch     USB keyboard enumerates again

The failure on 08dbfad3f504:

  xhci_hcd 0000:00:04.0: xHCI Host Controller
  xhci_hcd 0000:00:04.0: new USB bus registered, assigned bus number 1
  xhci_hcd 0000:00:04.0: Failed to allocate interrupter erst
  xhci_hcd 0000:00:04.0: can't setup: -12
  xhci_hcd 0000:00:04.0: USB bus 1 deregistered
  xhci_hcd 0000:00:04.0: init 0000:00:04.0 fail, -12
  xhci_hcd 0000:00:04.0: probe with driver xhci_hcd failed with error -12

Reproduced with:

  qemu-system-x86_64 -enable-kvm -m 512 -display none -serial stdio \
      -kernel bzImage -initrd initrd \
      -device qemu-xhci -device usb-kbd

Both -device qemu-xhci and -device nec-usb-xhci are affected. Reading
HCSPARAMS2 from inside the guest gives 0x0000000f, so ERST Max really is 0
and max_segs comes out as 0 with the current code.

I did not see the page allocator WARNING you hit (my .config is a minimal
tinyconfig-derived one), but the probe failure and the -12 are identical,
and this patch fixes it.

Tested-by: Pierre-David Belanger <pierredavidbelanger@gmail.com>

Thanks for the quick fix.

Pierre-David