Hi,

I'm trying to make mono output work on the audio driver for the Cirrus EDB9302 evaluation board. For some reason, plughw is refusing to do any channel conversion. When I try to play a mono file, I simply get:

aplay: set_params:1022: Unable to install hw params:

The driver's struct snd_pcm_hardware contains:

.channels_min = 2, .channels_max = 2,

which I believe is the correct way to tell userspace "this driver can only do stereo output" - at least, this is how intel8x0.c does it, and that driver really ought to work by now!

I've traced this to the exact point of failure, and what I see is that in snd_pcm_plug_hw_params(), despite figuring out exactly what the set of hardware parameters will work for the slave device, it goes and calls _snd_pcm_hw_params() with the actual parameters requested by the client, which obviously fails, and leads to the error shown above.

I added some calls to snd_pcm_hw_params_dump() which give this output. The first is sparams, which is the refined set of parameters which I assume are supposed to be set in the slave device. The second is params, which is the set of parameters requested by the client (aplay in this case).

sparams are: ACCESS: RW_INTERLEAVED FORMAT: U8 SUBFORMAT: STD SAMPLE_BITS: 8 FRAME_BITS: 16 CHANNELS: 2 RATE: 11025 PERIOD_TIME: (124988 124989) PERIOD_SIZE: 1378 PERIOD_BYTES: 2756 PERIODS: (4 5) BUFFER_TIME: (500045 500046) BUFFER_SIZE: 5513 BUFFER_BYTES: 11026 TICK_TIME: 0 params are: ACCESS: RW_INTERLEAVED FORMAT: U8 SUBFORMAT: STD SAMPLE_BITS: 8 FRAME_BITS: 8 CHANNELS: 1 RATE: 11025 PERIOD_TIME: (124988 124989) PERIOD_SIZE: 1378 PERIOD_BYTES: 1378 PERIODS: (4 5) BUFFER_TIME: (500045 500046) BUFFER_SIZE: 5513 BUFFER_BYTES: 5513 TICK_TIME: 0 inserting plugins whatever that means in snd_pcm_plug_change_channels!

So, yeah, that's great, it figured it all out, and realized that it had to do channel conversion, which presumably is done in snd_pcm_plug_change_channels(). But now, we see that it goes ahead and tries to set these parameters in the slave device:

calling _snd_pcm_hw_params with params: ACCESS: RW_INTERLEAVED FORMAT: U8 SUBFORMAT: STD SAMPLE_BITS: 8 FRAME_BITS: 8 CHANNELS: 1 RATE: 11025 PERIOD_TIME: (124988 124989) PERIOD_SIZE: 1378 PERIOD_BYTES: 1378 PERIODS: (4 5) BUFFER_TIME: (500045 500046) BUFFER_SIZE: 5513 BUFFER_BYTES: 5513 TICK_TIME: 0

But that's wrong! The slave device can't do single-channel input!

So what gives? This can't possibly be broken, or nobody's Intel on-board audio would ever work at all... so what is my driver doing wrong?