On Mon, 01 Mar 2021 16:24:00 +0100, Anton Yakovlev wrote: > > On 01.03.2021 15:56, Takashi Iwai wrote: > > On Mon, 01 Mar 2021 15:47:46 +0100, > > Anton Yakovlev wrote: > >> > >> On 01.03.2021 14:32, Takashi Iwai wrote: > >>> On Mon, 01 Mar 2021 10:25:05 +0100, > >>> Anton Yakovlev wrote: > >>>> > >>>> On 28.02.2021 12:27, Takashi Iwai wrote: > >>>>> On Sat, 27 Feb 2021 09:59:52 +0100, > >>>>> Anton Yakovlev wrote: > >>>>>> +/** > >>>>>> + * virtsnd_pcm_event() - Handle the PCM device event notification. > >>>>>> + * @snd: VirtIO sound device. > >>>>>> + * @event: VirtIO sound event. > >>>>>> + * > >>>>>> + * Context: Interrupt context. > >>>>> > >>>>> OK, then nonatomic PCM flag is invalid... > >>>> > >>>> Well, no. Here, events are kind of independent entities. PCM-related > >>>> events are just a special case of more generic events, which can carry > >>>> any kind of notification/payload. (And at the moment, only XRUN > >>>> notification is supported for PCM substreams.) So it has nothing to do > >>>> with the atomicity of the PCM device itself. > >>> > >>> OK, thanks. > >>> > >>> Basically the only question is how snd_pcm_period_elapsed() is called. > >>> And I see that it's called inside queue->lock, and this already > >>> invalidates the nonatomic PCM mode. So the code needs the fix: either > >>> fix this locking (and the context is guaranteed not to be an irq > >>> context), or change to the normal PCM mode without nonatomic flag. > >>> Both would bring some side-effect, and we need further changes, I > >>> suppose... > >> > >> Ok, I understood the problem. Well, I would say the nonatomic PCM mode > >> is more important option, since in this mode we can guarantee the > >> correct operation of the device. > > > > Which operation (except for the resume) in the trigger and the pointer > > callbacks need the action that need to sleep? I thought the sync with > > the command queue is done in the sync_stop. And most of others seem > > already taking the spinlock in themselves, so the non-atomic operation > > has little merit for them. > > All three commands from the beginning of that switch in trigger op: > ops->trigger(START) > ops->trigger(PAUSE_RELEASE) > ops->trigger(RESUME) > > They all result in > virtsnd_ctl_msg_send_sync(VIRTIO_SND_R_PCM_START) > > And that command must be sync, i.e. we need to wait/sleep until device > sent response. There are two major reasons for that: we need to be sure, > that substream has been actually started (i.e. device said OK). And we > need to take into account the virtualization overhead as well. Since > substream starting on device side may take some time, we can not > return from the trigger op until it actually happened. In atomic mode > all these nuances are lost, and it may lead to incorrect application > behavior. I see, then the nonatomic mode is the only way to go, indeed. Takashi
