linux-kernel - Re: [PATCH] Revert "PCI: Enable NVIDIA HDA controllers"

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Date:   Wed, 31 Jul 2019 23:26:39 +0200
From:   Karol Herbst <kherbst@...hat.com>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Lyude Paul <lyude@...hat.com>,
        nouveau <nouveau@...ts.freedesktop.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Daniel Drake <drake@...lessm.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Aaron Plattner <aplattner@...dia.com>,
        Peter Wu <peter@...ensteyn.nl>,
        Ilia Mirkin <imirkin@...m.mit.edu>,
        Maik Freudenberg <hhfeuer@....de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Revert "PCI: Enable NVIDIA HDA controllers"

On Wed, Jul 31, 2019 at 11:18 PM Lukas Wunner <lukas@...ner.de> wrote:
>
> On Wed, Jul 31, 2019 at 04:19:27PM -0400, Lyude Paul wrote:
> > While this fixes audio for a number of users, this commit has the
> > sideaffect of breaking the BIOS workaround that's required to make the
> > GPU on the nvidia P50 work, by causing the GPU's PCI device function to
> > stop working after it's been set to multifunction mode.
>
> This is missing a reference to the commit introducing the P50 quirk,
> which is e0547c81bfcf ("PCI: Reset Lenovo ThinkPad P50 nvgpu at boot
> if necessary").
>
> Please describe in more detail how the GPU's PCI function stops working.
> Does it respond with "all ones" when accessing MMIO?
> Do MMIO accesses cause the system to hang?
>
> Could you provide lspci -vvxx output for the GPU and its associated
> HDA controller with and without b516ea586d71?
>
> Does this machine have external display connectors via which audio
> can be streamed?
>
>
> > I'm not really holding my breath on this patch to being accepted:
> > there's a good chance there's a better solution for this (and I'm going
> > to continue investigating for one after sending this patch), this is
> > more just to start a conversation on what the proper way to fix this is.
>
> Posting as an RFC might have been more appropriate then.
>

no, a revert is actually appropriate.  If a commit fixes something,
but breaks something else, it gets either reverted or fixed. If nobody
fixes it, then revert it is.

>
> > So, I'm kind of confused about why exactly this was implemented as an
> > early boot quirk in the first place. If we're seeing the GPU's PCI
> > device, we already know the GPU is there. Shouldn't we be able to check
> > for the existence of the HDA device once we probe the GPU in nouveau?
>
> I think a motivation to keep this generic was to make it work with
> other drivers besides nouveau, specifically Nvidia's proprietary driver.
> nouveau might not even be enabled.
>
>
> > that still doesn't explain why this was implemented as an early quirk
>
> This isn't an early quirk.  Those live in arch/x86/kernel/early-quirks.c.
> This is just a PCI quirk executed on device enumeration and on resume.
> Devices aren't necessarily enumerated only on boot, e.g. think Thunderbolt.
>
> Thanks,
>
> Lukas