Re: [PATCH] ALSA: compress: allow pause and resume during draining

[Takashi Iwai <tiwai@suse.de>]

On Mon, 12 Oct 2020 16:10:10 +0200,
Jaroslav Kysela wrote:
> 
> Dne 12. 10. 20 v 15:55 Vinod Koul napsal(a):
> > On 12-10-20, 15:29, Jaroslav Kysela wrote:
> >> Dne 12. 10. 20 v 14:24 Vinod Koul napsal(a):
> >>> On 12-10-20, 09:01, Takashi Iwai wrote:
> >>>> On Mon, 12 Oct 2020 07:25:25 +0200,
> >>>
> >>>>> So what if we add another state but keep it in kernel (hidden from
> >>>>> userspace)...?
> >>>>
> >>>> That's fine, then it's just a kernel's business, and it should be
> >>>> determined which one makes the code better.
> >>>>
> >>>> But, there are things to be considered, though:
> >>>>
> >>>> - SNDRV_PCM_STATE_* is defined as snd_pcm_state_t with __bitwise.
> >>>>   This indicates that the type has to be defined in that way
> >>>>   explicitly.
> >>>>
> >>>> - Having a value over SNDRV_PCM_STATE_LAST internally is hackish.
> >>>>
> >>>>> Right now tinycompress does not make use of PCM streams, kernel handles
> >>>>> these. I am not aware of any other implementation.
> >>>>>
> >>>>> So if the scope if within compress then it might work...
> >>>>
> >>>> Yes.  But currently the API uses SND_PCM_* even for the compress
> >>>> stuff.  Changing this value means to have influence on PCM, even if
> >>>> PCM stuff doesn't use it yet.  (At least you'd need to increase
> >>>> SND_PCM_STATE_LAST, for example.)
> >>>>
> >>>> That said, if we want to change only for compress API by assuming that
> >>>> the impact must be negligible, the first step would be to move from
> >>>> SND_PCM_STATE_* to the own state, SND_COMPRESS_STATE_*.  The values
> >>>> should be compatible, but this has to be changed at first.  Then you
> >>>> can introduce a new value there.
> >>>
> >>> I think that sounds reasonable to me, we should not have used
> >>> SNDRV_PCM_STATE_* in the first place and long term fix for this should
> >>> be SNDRV_COMPRESS_STATE_
> >>>
> >>> I will cook a patch for this
> >>
> >> Although the impact is not high, I do think that we should enable the new
> >> behaviour conditionally (when the user space asks for it) even if the state
> >> values are split. I think that the whole thread is about 'how to extend the
> >> current APIs'. The hidden way is really not so nice.
> >>
> >> Unfortunately, there are no reserved fields in the snd_compr_params structure
> >> for this, but I see the similarity with the 'no_wake_mode' field which
> >> controls the driver behaviour.
> > 
> > I was not really thinking of exporting the states to userspace.
> > Tinycompress does not use it, I do not see any uses of it to enable
> > userspace with it.. Do you think it should be exposed? If so why..?
> 
> I don't think that it's required to expose the state for the compressed API to
> add this new feature. I just talk about to activate the new feature
> conditionally. The question is how to extend the API now.

The PCM API has an ioctl (SNDRV_PCM_IOCTL_USER_PVERSION) to tell
kernel which protocol version the user-space can talk with.  It's a
reverse direction from SNDRV_PCM_IOCTL_PVERSION, and with this
mechanism, the kernel can determine whether a specific feature can be
enabled to user-space or not.

I guess the compress API can introduce the same mechanism, if any
conditional behavior is really mandatory.

But, I doubt whether we really need to care about that; as mentioned
earlier, there is little to change from the user-space side.  It just
pause or resume.  The only difference is the resume target, and
honestly speaking, there is no interest in it from user-space side.
And, the rest is about the kernel internal, and this can be really
done in the way of the original patch.  The flow is quite simple and
understandable...


> > Worst case we add an ioctl to query the state.. the state transitions
> > are anyway result of control ops on the stream
> > 
> > Btw what was the motivation for pcm to expose the stream states..?
> 
> The driver may change the state when underrun / overrun or an I/O error occurs
> and there's also mmap write/read mode, so the traditional read/write with an
> error code handling does not work here. Also, the user space should know the
> state anyway, so it's better to have all parts synced.

For PCM, yes, the state query is a must from user-space applications,
and that's the reason I've been arguing.


thanks,

Takashi