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

[Jaroslav Kysela <perex@perex.cz>]

Dne 30. 09. 20 v 12:33 Takashi Iwai napsal(a):
> On Wed, 30 Sep 2020 11:57:45 +0200,
> Jaroslav Kysela wrote:
>>
>> Dne 30. 09. 20 v 11:35 Takashi Iwai napsal(a):
>>> On Tue, 29 Sep 2020 19:27:17 +0200,
>>> Jaroslav Kysela wrote:
>>>>
>>>> Dne 29. 09. 20 v 9:12 Takashi Iwai napsal(a):
>>>>> On Tue, 29 Sep 2020 03:51:35 +0200,
>>>>> Gyeongtaek Lee wrote:
>>>>>>
>>>>>> On 9/28/20 11:35 PM, Pierre-Louis Bossart wrote:
>>>>>>> On 9/28/20 6:13 AM, Jaroslav Kysela wrote:
>>>>>>>> Dne 28. 09. 20 v 12:50 Gyeongtaek Lee napsal(a):
>>>>>>>>> With a stream with low bitrate, user can't pause or resume the stream
>>>>>>>>> near the end of the stream because current ALSA doesn't allow it.
>>>>>>>>> If the stream has very low bitrate enough to store whole stream into
>>>>>>>>> the buffer, user can't do anything except stop the stream and then
>>>>>>>>> restart it from the first.
>>>>>>>>> If pause and resume is allowed during draining, user experience can be
>>>>>>>>> enhanced.
>>>>>>>>
>>>>>>>> It seems that we need a new state to handle the pause + drain condition for
>>>>>>>> this case.
>>>>>>>>
>>>>>>>> With this proposed change, the pause state in drain is invisible.
>>>>>>>
>>>>>>> Indeed it's be much nicer to have a new state, e..g 
>>>>>>> SNDRV_PCM_STATE_DRAINING_PAUSED.
>>>>>> Ok. I will add the new state.
>>>>>>>
>>>>>>> One concern is that states are defined in uapi/sound/asoc.h, so wouldn't 
>>>>>>> this have impacts on userspace as well? We'd change the value of 
>>>>>>> SNDRV_PCM_STATE_LAST.
>>>>>>>
>>>>>> I also agree that adding new state and increase LAST value in the header of uapi
>>>>>> could be dangerous. So, I added it to comress_offload.h for now.
>>>>>> It could be merged into snd_pcm_state_t in someday with big changes.
>>>>>> Could you review the fixed patch below?
>>>>
>>>> I don't see a big problem to improve the API, but don't forget to increase the
>>>> SNDRV_COMPRESS_VERSION, so the user space apps can check for this new behaviour.
>>>>
>>>>> Hrm, this resulted in rather more complex changes than the original
>>>>> patch.  It shows that introducing yet another state is no good idea
>>>>> for this particular case.
>>>>
>>>> I don't think so. The states should be isolated and it's clearly a new state
>>>> and the resulted code at least gives a commented idea, what's going on. It
>>>> seems that the compress driver state is not exported to the user space at the
>>>> moment, so I would consider this extension as harmless. We can add this state
>>>> to asound.h so the user space can be updated. We may use this state for the
>>>> standard PCM devices one day, too. It makes sense to reserve it sooner than later.
>>>
>>> Well, adding a new state can be cumbersome sometimes. For example, the
>>> code like below may hit a segfault out of sudden after the upgrade:
>>>
>>> 	const char *states[SNDRV_PCM_STATE_LAST + 1] = {
>>> 		[SNDRV_PCM_STATE_RUNNING] = "running",
>>> 		....
>>> 	};
>>>
>>> 	printf("current state = %s\n", states[s]);
>>>
>>> It's not much frequent breakage, but this can give certainly some
>>> incompatibilities even in the source code level.
>>
>> alsa-lib has already the correct protection for this case:
>>
>> const char *snd_pcm_state_name(const snd_pcm_state_t state)
>> {
>>         if (state > SND_PCM_STATE_LAST)
>>                 return NULL;
>>         return snd_pcm_state_names[state];
>> }
>>
>> If there's no check, it's a clear bug.
> 
> That's not what I meant; the code I showed was just an example
> implying that the addition of a new state may require the deep code
> change that can't be caught by a compiler.  It may be silently
> broken.
> 
> And imagine the user-space library code that contains handling of the
> PCM state.  All those has to be updated as well to deal with a new
> state, not only alsa-lib.
> 
> IOW, by adding a new item to an exposed attribute like PCM state, the
> possibly needed change would be spread over all lib / application
> code, and its influence shouldn't be underestimated.  If it were only
> some internal change in alsa-lib, I won't be concerned at all.
> 
>>> That's the reason I'm reluctant to add a new state unless it's a must.
>>> As mentioned, the expected application's behavior is just like the
>>> normal pause state, either resuming pause or dropping.  The only case
>>> where a new state would help for application is at most that they may
>>> foresee beforehand which state it'll go after the resume, to drain or
>>> to running.  If this is a must-to-have feature, we can reconsider.
>>
>> I don't agree here. It's much better to not hide the state related transitions
>> even in the kernel in my eyes. For example drivers may behave differently when
>> they resume from running+pause or drain+pause states.
> 
> Yes, but that's basically the driver's business.  As mentioned, "if
> this is a must-to-have feature" for applications, we'll need to
> reconsider.  But it's not clear from the scenario yet.
> (FWIW, if any, we may add another function to tell the after-resume
> state, too; this might be even safer from the compatibility POV,
> although it can be more complicated.)
> 
>> The correct SNDRV_PCM_STATE_LAST is just an implementation issue, which can be
>> easily solved.
> 
> How easily solvable -- that's the question :)

My proposal is reasonable - use the new state only internally in the kernel
for the moment, but update the headers and SNDRV_PCM_STATE_LAST now so the
depending code can be updated until the new state is exposed to the user
space, too. Something like future reservation. I believe that we need this
state also for the standard PCM API.

The SNDRV_PCM_STATE_LAST was added because it was supposed to be changed. It
great to keep the 100% driver compatibility but not if it forces us to do
hidden changes.

Another way is to activate the new state (and behaviour) conditionally using a
new parameter / flag or so from the user space. In this case, both sides know
what to do.

					Jaroslav

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