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. Furthermore, this also means that kernel implemenation such as ALSA sequencer also need to destroy/create rawmidi substream.
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.
I had considered about it when working for snd-firewire-digi00x (Mar 2015), and realised that it may take much time. At least, we work for:
- ALSA rawmidi core
- ALSA rawmidi interface for userspace
We don't need to extend rawmidi user-space API so much. Again, what needed is to return an error at read/write, probably -EPIPE, right after the driver got a hardware error. For that, just adding runtime->error flag and checking it appropriate should suffice (in theory). The read/write in rawmidi is pretty simple. And we don't have to reset the flag until the stream is reopened. There is no setup/preparation phase for rawmidi unlike PCM.
Additionally, users mostly use rawmidi devices via ALSA sequencer:
- ALSA sequencer core
- ALSA sequencer interface for userspace
Furthermore, regression test is required for:
- ALSA drivers with MIDI functionality
- ALSA rawmidi/sequencer applications in userspace
It's not 2000s. I guess that little developers are attracted for this work. In my option, the work is really cost-expensive than actual merit. So currently I'm negative for the idea.
A regression test is needed of course, but a simple error check can't regress except for the driver that really supports it.
Regards
Takashi Sakamoto