On Mon, May 11, 2009 at 11:58 AM, Takashi Iwai <tiwai@xxxxxxx> wrote: > At Mon, 11 May 2009 11:50:22 -0400, > Jon Smirl wrote: >> >> On Mon, May 11, 2009 at 11:40 AM, Takashi Iwai <tiwai@xxxxxxx> wrote: >> > At Mon, 11 May 2009 11:11:55 -0400, >> > Jon Smirl wrote: >> >> >> >> On Mon, May 11, 2009 at 9:45 AM, Jaroslav Kysela <perex@xxxxxxxx> wrote: >> >> > On Mon, 11 May 2009, Jon Smirl wrote: >> >> > >> >> >>> Right. This is the value to check in your case. >> >> >> >> >> >> What do think about redesigning the ALSA DMA interface to support >> >> >> detection of over and under run? Leaving the DMA engine in a loop and >> >> >> not coordinating with ALSA as to where the valid data is does not seem >> >> >> to be a safe way of exchanging data. That interface may be a source of >> >> >> the problems pulseaudio is encountering. >> >> >> >> >> >> A simple solution would be for snd_pcm_period_elapsed() to return >> >> >> physical address of the last valid sample. That would let me avoid >> >> >> playing with s->runtime->control->appl_ptr. You could provide the >> >> >> same data in the pointer() function. >> >> > >> >> > More simpler solution is to check the stream state in the low level driver. >> >> > If it's in DRAINING state, then end of stream is signaled from the >> >> > application and driver might not queue next buffer. We may also add another >> >> > callback (or use ioctl callback) to pass this stream state change to the >> >> > lowlevel driver immediately, so the driver might react more quickly on this >> >> > situation. >> >> > >> >> >> >> Quickness is the wrong way to think about this problem. ALSA knows exactly >> >> when it has placed valid data into the buffer. >> > >> > Not really. When the mmap mode is used, the update isn't always >> > notified to the driver and the transfer can be completely >> > asynchronous. >> >> This seems to me to be a broken design. ALSA is being put into the >> position of guessing when the application has supplied new data. >> Shouldn't the app be required to make a commit() call after filling in >> the data? Without commit it is impossible to detect over/underrun. > > The commit updates the mmapped control data (so that it works even > without the context switch) if the architecture supports. In other > cases, a commit issues an explicit sync ioctl. > > Actually it should be possible to disable the mmap-control mode > explicitly, but right now it's not done from the driver side but only > checks the architecture. Shared memory is another solution that doesn't involve context switches. The app can update it's valid pointer in shared memory. My IRQ will call snd_pcm_period_elapsed(). snd_pcm_period_elapsed() can find the updated valid pointer, convert it to a physical address and leave it in a shared structure. When snd_pcm_period_elapsed() returns, my IRQ can get to the pointer and submit the necessary buffers. What's missing is an official way of accessing s->runtime->control->appl_ptr from the low level driver. We're implementing a ring buffer. In a ring buffer I have to know where both pointers are in order to detect over/under run. I also don't understand why this is specific to my hardware, every DMA implementation should need these two pointers. > > > Takashi > -- Jon Smirl jonsmirl@xxxxxxxxx _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
