Re: [PATCH v3 3/3] ALSA: hda: Disabled unused audio controller for Dell platforms with Switchable Graphics

[Lukas Wunner <lukas@wunner.de>]

On Mon, Apr 16, 2018 at 04:25:12PM +0200, Pali Rohár wrote:
> On Sunday 15 April 2018 21:05:23 Lukas Wunner wrote:
> > On Sun, Apr 15, 2018 at 07:17:46PM +0200, Pali Rohár wrote:
> > > On Saturday 14 April 2018 13:17:11 Lukas Wunner wrote:
> > > > On Sat, Apr 14, 2018 at 12:49:50PM +0200, Pali Rohár wrote:
> > > > > On Saturday 14 April 2018 12:45:12 Lukas Wunner wrote:
> > > > > > On Thu, Apr 12, 2018 at 10:15:41PM +0800, Kai-Heng Feng wrote:
> > > > > > > Do you have any suggestion to check if it connects to the system via
> > > > > > > Thunderbolt?
> > > > > > 
> > > > > > Just use pci_is_thunderbolt_attached(), introduced by 8531e283bee6,
> > > > > > like this:
> > > > > > 
> > > > > > if (check_dell_switchable_gfx(pci) && !pci_is_thunderbolt_attached(pci))
> > > > > 
> > > > > And what about PCI-e device attached to ExpressCard slot?
> > > > 
> > > > I don't know of a bullet-proof way to recognize those.
> > > 
> > > Hm... maybe another idea: Is it possible to detect which audio pci
> > > device belongs to graphics card via vga_switcheroo? Currently, looking
> > > at output it is same PCI device as graphic card, just different PCI
> > > function.
> > 
> > No, the DRM drivers don't filter ExpressCard-attached GPUs when
> > registering with vga_switcheroo.
> 
> > They do filter Thunderbolt-attached GPUs.
> 
> So check via vga_switcheroo should at least work for Thunderbolt GPUs.
> 
> I do not know if it is possible, but for me it looks like that check via
> vga_switcheroo should be better then adding another heuristic to other
> drivers.

It is way simpler and more straightforward if hda_intel.c calls
pci_is_thunderbolt_attached() directly, as I've suggested above.

The DRM drivers call that as well and register with vga_switcheroo
only if it returns false.  So if hda_intel.c asked vga_switcheroo
and got back a negative answer, it would never know whether it's
because the DRM driver hasn't finished probing yet, or it's module
is blacklisted, or whatever.


> And once we would have good/reliable check for EC devices we can add it
> into vga_switcheroo and all users of it could benefit. Anyway, I think
> that vga_switcheroo should filter also EC GPU cards if it already
> filters Thunderbolt.

vga_switcheroo doesn't do the filtering, the DRM drivers themselves
decice whether to register with vga_switcheroo or not.  The motivation
is to avoid middle layers and instead provide the DRM drivers with
a library of helpers that they may call at their own discretion.

See this blog post and the links therein for background info:
http://blog.ffwll.ch/2016/12/midlayers-once-more-with-feeling.html

Thanks,

Lukas