[alsa-devel] [PATCH 3/3] ALSA: pcm: conditionally avoid mmap of control data

[Takashi Sakamoto]

On May 16 2017 14:46, Takashi Iwai wrote:
> In this case, the problem is that the mmap control allows the appl_ptr
> being changed silently without interaction with the driver.  If the
> driver requires some explicit action for changing the appl_ptr, it
> won't work.

I think 'struct snd_pcm_ops.pointer' is available for this purpose, too.

static snd_pcm_sframes_t snd_pcm_playback_rewind(...)
{
     ...
     hw_avail = snd_pcm_playback_hw_avail(runtime);
     (->struct snd_pcm_ops.pointer())
     ...
     (rewind PCM frames)
     ...
+   substream->ops->pointer(...);
     (->struct snd_pcm_ops.pointer())
     ...
}

If drivers need to handle event to update the appl_ptr, it records value 
of the appl_ptr, then compare it to current value to get the updates.

I note that the callback is done in both of process/irq contexts.

On May 16 2017 14:46, Takashi Iwai wrote:
 > Worthless is a too strong word, but I agree that more clarification
 > would be more helpful.

Sorry to use such strong expression. I'll care of it.


Regards

Takashi Sakamoto