[alsa-devel] [PATCH] ALSA: cmipci: Allow disabling MPU port via module option

[Clemens Ladisch]

Takashi Iwai wrote:
> Patrick May reported that his sound card with CMI8378 chip causes a
> crash / reboot when accessing the MIDI port that isn't actually
> present on the board.  [...]
> Although fixing the unexpected reboot would be the best solution, it's
> not so trivial to identify the cause.

There was a similar problem in the CMI8788 driver:
https://git.kernel.org/linus/f1bc07af9a9e
https://bugtrack.alsa-project.org/alsa-bug/view.php?id=4496 (does this still exist?)

When reading a register repeatedly, the chip sometimes does not
complete the PCI transaction.  Apparently, mainboard PCI chipsets
ignore this error, but PCIe/PCI bridges generate a machine check
exception in this case.

When writing MIDI data to a MPU-401-like device, the driver regularly
polls the status register.  I expect that this will blow up with any
C-Media chip on a card with a PCIe/PCI bridge, regardless of whether
a MIDI port exists.

(I wonder if this can be triggered with snd_pcm_avail() in a loop.
Maybe it happens only for byte accesses, or nobody has yet found out.)

> As default, the MPU port is still enabled

It might be a good idea to disable it by default if the parent
bridge is an ASM1083.


Regards,
Clemens