[PATCH v2 1/3] ALSA: pcm: introduce INFO_NO_REWINDS flag

[Pierre-Louis Bossart]

On 10/5/21 1:59 AM, Takashi Iwai wrote:
> On Mon, 04 Oct 2021 18:24:21 +0200,
> Pierre-Louis Bossart wrote:
>>
>> When the hardware can only deal with a monotonically increasing
>> appl_ptr, this flag can be set. In case the application requests a
>> rewind, snd_pcm_rewind() will not return an error code but signal that
>> the appl_ptr was not modified.
> 
> This modification itself is fine, but I guess that application may
> still move the appl_ptr directly via SNDRV_PCM_IOCTL_SYNC_PTR ioctl.
> We need to verify the backward move there, I suppose?

Sorry Takashi, I wasn't able to fully follow your question.

In the previous version, I had an explicit check to see if the appl_ptr
was modified by a rewind, but you mentioned this would be broken for
32-bit devices due to the use of the 'boundary'. I really have no idea
how we can detect a rewind in this configuration, so  if you are asking
to detect when the appl_ptr is modified through some other means (which
I didn't get) we will have the same problem, won't we?

see the initial thread here:

https://lore.kernel.org/alsa-devel/de5e91c6-5f0e-9866-a1c2-0943b4342359@linux.intel.com/