[alsa-devel] [RFC PATCH] ALSA: hda - Implement a poll loop for jacks as a module parameter

David Henningsson david.henningsson at canonical.com
Wed Oct 10 15:47:31 CEST 2012

On 10/09/2012 04:57 PM, Takashi Iwai wrote:
> At Tue, 09 Oct 2012 16:28:07 +0200,
> David Henningsson wrote:
>> On 10/09/2012 03:32 PM, Takashi Iwai wrote:
>>> At Tue,  9 Oct 2012 15:19:44 +0200,
>>> David Henningsson wrote:
>>>> Now that we have a generic unsol mechanism, we can implement a generic
>>>> poll loop, which can be used for debugging, or if a codec's unsol
>>>> mechanism is broken.
>> Oh, and I was also considering not turning on unsol at all (in
>> snd_hda_jack_detect_enable) when this is enabled. Then it would help
>> against "unstable" pins which trigger repeatedly on/off even though
>> nothing is happening physically.
> In that case, it's fine.  But it's no broken codec's unsol mechanism.
> It's a broken jack detection in hardware, i.e. it's no fault of
> codec.

Ok, will send an additional patch for that once this one is applied.

>>>> Signed-off-by: David Henningsson <david.henningsson at canonical.com>
>>> The patch almost looks good, but I postpone as a 3.8 thing.
>> Ok. I never understood the release timings, but I figured anything
>> before rc1 would be okay to merge as a feature...
> No, basically when the merge window opens, all new features should be
> frozen.  I took a few your patches yesterday just because I've been
> traveling in the last weeks, and the generic unsol event code itself
> is mostly nothing but a code shuffling from here to there.
>> (And I couldn't start
>> a week earlier; because I didn't know if you would accept the generic
>> unsol event.)
>> That said, it's not very important to me personally which kernel this
>> goes into.
> OK.
>>>> I'm also considering activating it by default if we go into single_cmd mode
>>>> due to codec not responding. What do you think?
>>> Well, I don't buy it.  Falling back to single_cmd doesn't mean that we
>>> are saving the world perfectly.I recently even think it might be
>>> even better not to do that, otherwise people won't notice the
>>> breakage.
>> This is almost philosophical, but if a bug occurs and no user notices,
>> is it really a bug? :-)
> Yes, it performs worse secretly.  The fallback to single_cmd is really
> a last resort and it's no peaceful place to live at all.

If that is your real thinking, maybe we should make the warning message 
reflect that:

	snd_printk(KERN_ERR "hda_intel: azx_get_response timeout, "
		   "switching to single_cmd mode! This performs worse "
		   "secretly, is not saving the world perfectly, and "
		   "it's no peaceful place to live at all!\n");

>> Or, to make it a bit more pragmatic, what other things are broken with
>> single_cmd? Could you give an example where this change would be harmful?
> The single cmd mode itself was introduced as a sort of rescue command
> without CORB/RIRB.  We shouldn't use it in normal situations.  It's
> simply no considered use case.
> With a lack of unsol event, you can't handle the volume knob, or any
> GPIO events, too.

Those two are quite uncommon, and can be polled too if necessary.

>>>> +			codec->jackpoll_interval =
>>>> +				msecs_to_jiffies(jackpoll_ms[chip->dev_index]);
>>> Better to add a sanity check of the interval value.
>> Ok. Attaching modified patch.
> Well, do you want to allow 1ms polling?

I can't see why not, if people want to burn CPU cycles, why not let 
them. However, the real limit should probably be one jiffy, so attaching 
modified patch.
If you think there should be another lower limit, feel free to adjust 
the patch before applying. It's no big deal.

David Henningsson, Canonical Ltd.
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