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Wed, 30 Sep 2020 13:23:01 +0200 (CEST) Subject: Re: [PATCH] ALSA: compress: allow pause and resume during draining To: Takashi Iwai References: <000c01d69585$228db6b0$67a92410$@samsung.com> <7ba714ce-8b33-1b64-7503-6b155bf43909@perex.cz> <000f01d69603$10573fb0$3105bf10$@samsung.com> From: Jaroslav Kysela Message-ID: Date: Wed, 30 Sep 2020 13:23:01 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Cc: alsa-devel@alsa-project.org, khw0178.kim@samsung.com, lgirdwood@gmail.com, kimty@samsung.com, s47.kang@samsung.com, 'Pierre-Louis Bossart' , tiwai@suse.com, vkoul@kernel.org, hmseo@samsung.com, Gyeongtaek Lee , pilsun.jang@samsung.com, tkjung@samsung.com X-BeenThere: alsa-devel@alsa-project.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: "Alsa-devel mailing list for ALSA developers - http://www.alsa-project.org" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: alsa-devel-bounces@alsa-project.org Sender: "Alsa-devel" 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 Linux Sound Maintainer; ALSA Project; Red Hat, Inc.