linux-kernel - Re: [PATCH] ALSA: hda - Defer probe when loading patch firmware

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Date:	Thu, 9 Aug 2012 10:07:13 +0200
From:	Thierry Reding <thierry.reding@...onic-design.de>
To:	Takashi Iwai <tiwai@...e.de>
Cc:	Jaroslav Kysela <perex@...ex.cz>,
	David Henningsson <david.henningsson@...onical.com>,
	alsa-devel@...a-project.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ALSA: hda - Defer probe when loading patch firmware

On Thu, Aug 09, 2012 at 09:42:48AM +0200, Takashi Iwai wrote:
> At Thu, 9 Aug 2012 09:36:42 +0200,
> Thierry Reding wrote:
> > 
> > On Thu, Aug 09, 2012 at 09:31:30AM +0200, Takashi Iwai wrote:
> > > At Thu, 9 Aug 2012 09:08:13 +0200,
> > > Thierry Reding wrote:
> > > > 
> > > > On Thu, Aug 09, 2012 at 08:57:13AM +0200, Takashi Iwai wrote:
> > > > > At Thu,  9 Aug 2012 08:45:23 +0200,
> > > > > Thierry Reding wrote:
> > > > > > 
> > > > > > Recent changes to the firmware loading helpers cause drivers to stall
> > > > > > when firmware is loaded during the module_init() call. The snd-hda-intel
> > > > > > module requests firmware if the patch= parameter is used to load a patch
> > > > > > file. This patch works around the problem by deferring the probe in such
> > > > > > cases, which will cause the module to load successfully and the driver
> > > > > > binding to the device outside the module_init() call.
> > > > > 
> > > > > Is the "recent" change meant 3.6 kernel, or in linux-next?
> > > > > 
> > > > > In anyway, I don't understand why such a change was allowed.  Most
> > > > > drivers do call request_firmware() at the device probing time.
> > > > > If this really has to be resolved in the driver side, it must be a bug
> > > > > in the firmware loader core code.
> > > > 
> > > > A good explanation of the problem and subsequent discussion can be found
> > > > here:
> > > > 
> > > > 	http://article.gmane.org/gmane.linux.drivers.video-input-infrastructure/49975
> > > 
> > > Yeah, but it doesn't justify this ugly module option.
> > > It's a simple bug.  Papering over it with this option doesn't fix
> > > anything.
> > 
> > It's not an option, all it does is defer probing if and only if the
> > patch parameter was specified to make sure the firmware load won't
> > stall. I realize that this may not be an optimal solution, but at least
> > it fixes the problem with no fallout.
> 
> Ah sorry, I misread the patch.
> 
> Then it shouldn't be checked at that point.  Since 3.5 kernel, the
> probing code was already split for vga_switcheroo support.

Yes, I saw that. But unless you actually use vga_switcheroo, the second
stage, azx_probe_continue(), will still be called from azx_probe() and
therefore ultimately from module_init().

Before coming up with this patch I actually did play around a bit with
using the asynchronous firmware load functions but it turned out to be
rather difficult to do so I opted for the easy way. The biggest problem
I faced was that since patch loading needs to be done very early on, a
lot of the initialization would need to be done after .probe() and many
things could still fail, so cleaning up after errors would become
increasingly difficult.

> The point you added is the second stage.

I don't understand this sentence.

Thierry

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