[PATCH] ALSA: compress: allow pause and resume during draining

[Jaroslav Kysela]

Dne 09. 10. 20 v 17:13 Takashi Iwai napsal(a):
> On Thu, 08 Oct 2020 11:49:24 +0200,
> Gyeongtaek Lee wrote:
>>
>> On 10/06/20 11:57 PM, Pierre-Louis Bossart wrote:
>>>> The SM in kernel might be bit more convoluted so was wondering if we can
>>>> handle this in userland. The changelog for this patch says that for
>>>> test case was sending whole file, surely that is not an optimal approach.
>>>
>>> It's rather common to have to deal with very small files, even with PCM, 
>>> e.g. for notifications. It's actually a classic test case that exposes 
>>> design issues in drivers, where e.g. the last part of the notification 
>>> is not played.
>>>
>>>> Should we allow folks to send whole file to kernel and then issue
>>>> partial drain?
>>>
>>> I don't think there should be a conceptual limitation here. If the 
>>> userspace knows that the last part of the file is smaller than a 
>>> fragment it should be able to issue a drain (or partial drain if it's a 
>>> gapless solution).
>>>
>>> However now that I think of it, I am not sure what happens if the file 
>>> is smaller than a fragment. That may very well be a limitation in the 
>>> design.
>>>
>> Thanks for the comments.
>>
>> Actually, problem can be occurred with big file also.
>> Application usually requests draining after sending last frame.
>> If user clicks pause button after draining, pause will be failed
>> and the file just be played until end.
>> If application stop and start playback for this case, 
>> start of last frame will be heard again because stop sets state to SETUP,
>> and write is needed to set the state to PREPARED for start.
>> If bitrate of the file is low, time stamp will be reversed and be heard weird.
>> I also hope this problem can be handled in userspace easily but I couldn't find a way for now.
>>
>> I think that this is the time that I should share fixed patch following the comments to help the discussion.
>> Following opinions are added to the patch.
>> 1. it's be much nicer to have a new state - Takashi
> 
> Well, it wasn't me; I'm not against the new state *iff* it would end
> up with cleaner code.  Admittedly, the new state can be more
> "consistent" regarding the state transition.  If we allow the PAUSE
> state during DRAINING, it'll lead to multiple states after resuming
> the pause.
> 
>> 2. We can add this state to asound.h so the user space can be updated. - Jaroslav
>> 3. don't forget to increase the SNDRV_COMPRESS_VERSION - Jaroslav
>>
>> I'm bit wondering whether it is good to increase SNDRV_COMPRESS_VERSION
>> with a change in asound.h not in compress_offload.h.
>> Should I increase SNDRV_PCM_VERSION also?
> 
> Yes, if we really add the PCM state, it's definitely needed.
> 
>> And what happened if I request back-porting a patch which changes VERSION to stables?
> 
> If we introduce the new change, it must be safe to the old kernels,
> too.  The problem is only about the compatibility of the user-space
> program, not about the kernel.
> 
> 
> HOWEVER: I'm still concerned by the addition of a new PCM state.
> Jaroslav suggested two steps approach, (1) first add the state only in
> the uapi header, then use (2) the new state actually.  But, this
> doesn't help much, simply because the step 1 won't catch real bugs.
> 
> Even if we add a new state and change the SNDRV_PCM_STATE_LAST, I
> guess most of code can be compiled fine.  So, at the step 1, no one
> notices it and bothered, either.  But, at the step 2, you'll hit a
> problem.
> 
> Adding a new state is something like to add a new color to the traffic
> signal.  In some countries, the car turning right at a crossing
> doesn't have to stop at a red signal.  Suppose that we want to control
> it, and change the international rule by introducing a new color (say
> magenta) signal to stop the car turning right.  That'll be a big
> confusion because most drivers are trained for only red, green and
> yellow signals.
> 
> Similarly, if we add a new PCM state, every program code that deals
> with the PCM state may be confused by the new state.  It has to be
> reviewed and corrected manually, because it's no syntax problem the
> compiler may catch.

If there is a handshake between both side, this problem is gone. We can just
add another flag / ioctl / whatever to activate the new behaviour.

						Jaroslav

-- 
Jaroslav Kysela <perex at perex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.