[alsa-devel] RawMIDI behaviour with MidiFace 4x4

[Clemens Ladisch]

Kalvas, Taneli wrote:
>>> I have tested the system by connecting the output port to an input
>>> port on the hardware and having another program output the received
>>> bytes on stdout. My observations:
>>> - As long as the buffer size is 276 bytes or less it gets sent ok
>>> - Larger buffer sizes cause corruption and roughly 300 bytes of received.
>>
>> This sounds as if the device has a buffer size of 256 bytes, but does
>> not actually check if the buffer is full, and always accepts more data.
>>
>> Does later data overwrite data that should have been sent earlier?
>
> By looking at the receiving end bytes it looks exactly like that has happened.

This pretty much proves that this is a bug in the device firmware.

You might try contacting Miditech ...

>>> Is there a way to connect two programs via a virtual RawMIDI port so
>>> that I could test without using the hardware?
>>
>> Load the snd-virmidi module, and connect two of its ports with aconnect.
>
> I did this and by using the virtual ports the behaviour is more
> reasonable, but still not as expected. Everything works ok as long as
> the message length is less than or equal to the buffer size (4096).
> With message sizes larger than that the writing end still claims it
> has sent everything, as it should do because the port is opened in
> blocking mode. On the reading end I can see that some data is missing
> and xruns are reported.

Oops, using snd-virmidi this way also goes through the sequencer, which
does not do blocking.

There is no other way to get a virtual RawMIDI port.

However, the driver was tested with lots of other devices, and huge
SysEx messages have this problem only with your device.


Regards,
Clemens