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