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

[Takashi Iwai <tiwai@suse.de>]

At Thu, 9 Aug 2012 12:34:30 +0200,
Thierry Reding wrote:
> 
> On Thu, Aug 09, 2012 at 10:21:15AM +0200, Takashi Iwai wrote:
> > At Thu, 9 Aug 2012 10:07:13 +0200,
> > Thierry Reding wrote:
> > > 
> > > 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().
> > 
> > Yeah, but this could be easily delayed.  The split was already done,
> > so the next step would be to return after the first half at probe,
> > then call the second half later.
> > 
> > > 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.
> > 
> > async probe is also on my TODO list, but it's deferred ;)
> > 
> > > > The point you added is the second stage.
> > > 
> > > I don't understand this sentence.
> > 
> > I meant that your patch added the check at the second-half probing
> > function (azx_probe_contine()).  That is, it could be already the
> > point triggered by vga_switcheroo handler, not via module_init any
> > longer.
> > 
> > So, after rethinking what you suggested, I wrote a quick patch below.
> > Could you check whether this works?
> 
> It oopses, though I can't quite tell where. I need to test some more
> later to see where it goes wrong.

Yeah, I tested it here and noticed, too.  As mentioned, the behavior
of -EPROBE_DEFER is somehow flaky.  For example, when it's used for
modules, the deferred probe will be never triggered (unless a new
device is bound).

Considering the problem again, it's currently an issue only for the
built-in sound driver, right?  AFAIK, request_firmware() works fine
for modules.  If so, a simple "fix" to avoid the unexpected behavior
is to make CONFIG_SND_HDA_PATCH_LOADER depending on CONFIG_SND_HDA=m.
It'd be simple enough for merging 3.6 kernel.

I almost finished writing the patch to use request_firmware_nowait()
version, but I'm afraid it's too intrusive for 3.6.  If disabling the
patch loader for the built-in driver is OK, I'd queue the
request_firmware_nowait() patches to 3.7 queue.


Takashi