[alsa-devel] [PATCH 2/2] ALSA: oxfw: discontinue MIDI substream for scs1x at transaction failure

[Takashi Iwai]

On Fri, 19 Feb 2016 16:51:55 +0100,
Takashi Sakamoto wrote:
> 
> On Feb 19 2016 18:46, Takashi Iwai wrote:
> > On Fri, 19 Feb 2016 10:23:36 +0100,
> > Takashi Sakamoto wrote:
> >>
> >> Hi,
> >>
> >> On Feb 19 2016 17:19, Takashi Iwai wrote:
> >>> On Fri, 19 Feb 2016 01:55:50 +0100,
> >>> Takashi Sakamoto wrote:
> >>>>
> >>>> With a previous commit, ALSA oxfw driver retries transferring MIDI
> >>>> messages at transaction failure for scs1x. On the other hand, there're
> >>>> fatal transaction error. Then, MIDI messages never reach to the unit
> >>>> anymore. In this case, MIDI substream should be discontinued.
> >>>>
> >>>> This commit stops MIDI transferring after the fatal error occurs.
> >>>> Unfortunately, unlike ALSA PCM functionality, ALSA rawmidi core has no
> >>>> feature to discontinue MIDI substream in kernel side, thus this commit
> >>>> just stops MIDI transferring without notifying it to userspace.
> >>>
> >>> It's fine to take this, and I would take it as is for now.
> >>
> >> OK.
> >>
> >>> But we can extend the rawmidi somehow to deal with such an error, too.
> >>> Maybe just having "error" flag in the rawmidi runtime and adding a
> >>> helper function to set the error and stop the stream should work
> >>> easily.
> >>
> >> You forgot ALSA sequencer.
> > 
> > This is a different layer.  The sequencer stuff may check the error
> > and handle more gracefully than as of now, but it's a different story
> > from the extension of rawmidi itself.
> > 
> > What you're working on is the rawmidi, and what it can give is just
> > returning an error at the right moment.
> 
> I don't think so. I think the work is heavier than your expectation.
> 
> I pushed 'rawmidi-epipe' branch to my repository.
> https://github.com/takaswie/sound/tree/rawmidi-epipe
> 
> You can see four commits just to show my concerns.
> (They're not tested yet. Don't use them for actual work ;)
> 41f499b ALSA: rawmidi: add a helper to set runtime error
> 4164edb ALSA: rawmidi: handle EPIPE
> 5e7348ed ALSA: seq: handle EPIPE for rawmidi input
> 58d9008 ALSA: seq: handle EPIPE for rawmidi output
> 
> In the top-most commit, you can see userspace applications need to
> close/open character devices to recover from EPIPE state of rawmidi
> substream runtime.

Yes, this doesn't change from the current situation.

Remember that rawmidi may return an error already in the current
implementation.  The new feature would be just to add the explicit
trigger of the error, instead of implicit runtime->avail check.

> Furthermore, this also means that kernel
> implemenation such as ALSA sequencer also need to destroy/create rawmidi
> substream.

Only if sequencer layer matters.  Currently it just ignores.

> The main reason of them is ALSA rawmidi core has no functionality for
> recovery feature. In ALSA PCM core, 'struct snd_pcm_ops.prepare' has a
> role for it.

It'll never be, I guess.  The reopen is the simplest solution for
devices like rawmidi, and we have no big reason to make things
complicated.


Takashi