linux-kernel - Re: [PATCH] BUG-REPORT: snd-hda: hacked-together EPROBE

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Date:   Wed, 21 Jun 2017 16:23:57 +0100
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Daniel Vetter <daniel.vetter@...ll.ch>,
        "Intel Graphics Development" <intel-gfx@...ts.freedesktop.org>
Cc:     "Daniel Vetter" <daniel.vetter@...ll.ch>,
        "Jaroslav Kysela" <perex@...ex.cz>,
        "Takashi Iwai" <tiwai@...e.com>,
        "Guneshwor Singh" <guneshwor.o.singh@...el.com>,
        "Hardik T Shah" <hardik.t.shah@...el.com>,
        "Julia Lawall" <Julia.Lawall@...6.fr>,
        "Vinod Koul" <vinod.koul@...el.com>,
        "Subhransu S. Prusty" <subhransu.s.prusty@...el.com>,
        "Libin Yang" <libin.yang@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] BUG-REPORT: snd-hda: hacked-together EPROBE_DEFER support

Quoting Daniel Vetter (2017-06-21 16:08:54)
> So back when the i915 power well support landed in
> 
> commit 99a2008d0b32d72dfc2a54e7be1eb698dd2e3bd6
> Author: Wang Xingchao <xingchao.wang@...ux.intel.com>
> Date:   Thu May 30 22:07:10 2013 +0800
> 
>     ALSA: hda - Add power-welll support for haswell HDA
> 
> the logic to handle the cross-module depencies was hand-rolled using a
> async work item, and that just doesn't work.
> 
> The correct way to handle cross-module deps is either:
> - request_module + failing when the other module isn't there
> 
> OR
> 
> - failing the module load with EPROBE_DEFER.
> 
> You can't mix them, if you do then the entire load path just
> busy-spins blowing through cpu cycles forever with no way to stop
> this.
> 
> snd-hda-intel does mix it, because the hda codec drivers are loaded
> using request_module, but the i915 depency is handled using
> PROBE_DEFER (or well, should be, but I haven't found any code at all).
> This is a major pain when trying to debug i915 load failures.
> 
> This patch here is a horrible hackish attempt at somewhat correctly
> wriing EPROBE_DEFER through. Stuff that's missing:
> - Check all the other places where load errors are conveniently
>   dropped on the floor.
> - Also fix up the firmware_cb path.
> - Drop the debug noise I've left in to make it clear this isn't
>   anything for merging.

This tames "hdaudio hdaudioC0D0: Unable to bind the codec" which was
continuously spewing previously, and now the system is usable again.
Thanks,
-Chris