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

[Pierre-Louis Bossart]

> And, what I meant in the previous thread was that the check in the
> given patch wasn't "enough", i.e. it needs more careful checks
> considering the boundary crossing.  That is, you can't simply compare
> appl_ptr vs old_appl_ptr as a single condition for the backward move.

Indeed, that's why I tried to avoid any checks on pointers :-)

> For example, check snd_pcm_playback_avail() and co.  That contains a
> couple of more condition checks and corrections due to the possible
> boundary crossing.  (Here, runtime->boundary may differ depending on
> 32 or 64bit context.)
> 
> The actual implementation of the backward move check would be slightly
> different from those, but I hope you get my idea.

I think I do but not sure how to precisely deal with the boundary
wrap-around.

The only suggestion I have at this point would be to compare the 'avail'
space before and after the appl_ptr changes in pcm_lib_apply_appl_ptr().
If the 'avail' space grows as a result of user-space changes, that
indicates a rewind (both for capture and playback), doesn't it?

A tentative solution is shared here:
https://github.com/thesofproject/linux/pull/3207