[Nouveau] [Bug 75985] [NVC1] HDMI audio device only visible after rescan

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=75985

--- Comment #22 from Daniel Drake <dan at reactivated.net> ---
Created attachment 136369
  --> https://bugs.freedesktop.org/attachment.cgi?id=136369&action=edit
GP104: enable HDMI audio device function

On Asus GL502VS I investigated why the gfx device must be removed before rescan
in the above workarounds.

The reason is that when Linux first probes the device (before you attempt the
workaround), pci_setup_device() notes that the device is not multifunction
capable. This causes pci_scan_slot() to not bother scanning the non-zero
functions (per the behaviour inside next_fn()).

I checked and I found that when the 0x488 magic bit is not set, the gfx device
advertises as non-multifunction. After the bit is set, the device advertises as
multi-function. So, after setting the magic bit, removing the device will cause
Linux to re-probe it during the next rescan, taking note at that point that it
is a multi-function device, and proceeding to scan the functions, finding the
audio device at function 1.

Based on that I have a first attempt at a fix. It's not working though, audio
output is silent (but I did have it working with the previous workarounds).
I'll look closer next week.

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