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 :) thanks, Takashi