sw_params for a direct-ed(dmix) hw pcm

[sylvain.bertrand at gmail.com]

On Thu, Mar 26, 2020 at 03:36:23PM +0100, Jaroslav Kysela wrote:
> I agree. Also, the snd_pcm_direct_sw_params() does nothing, because the
> sw_params are already cached in the pcm structure (see comment). It means
> that the dmix (direct) plugins operates with those cached values. Just set
> sw_params like for any other PCM handle. The dmix uses those values (if
> possible).

This is the "if possible" which would impacts the way how code should do setup
right, but:

Let's take the case of a classic plugin "pipeline":
pcm:plug->...->direct::dmix->hw

>From the top plugin (usually plug) to the direct::plugin, the "sw_params" pcm
op is usually pcm_generic.c:snd_pcm_generic_sw_params which does recurse down.
This recursion down will stop once pcm_direct.c:snd_pcm_direct_sw_params is
reached, then will recurse up, without error.

But pcm.c:snd_pcm_sw_params will copy anyway the provided sw_params into each
recursed back pcm if the "sw_params" pcm op return no error code, which is the
case.

Then looking at pcm.c:snd_pcm_sw_params_current, I get those "wrong" sw_params,
then I get no way to know something went wrong.

Why "wrong", because they may significantly differ from the bottom hw plugin
sw_params which some fields are used to configure the kernel driver.

for instance, a fast_op status call will recurse down to
pcm_dmix.c:snd_pcm_dmix_status, which will call the hw plugin fast op status
function which will use _its_ tstamp_type field for the ioctl call, but will
"override" the trigger_tstamp field computed with the "wrong" sw_params
tstamp_type!

It happens that the monotonic_raw and monotonic clocks can have audio
significant difference. Additionally, the other sw_params field might cause
similar issues.

regards,

-- 
Sylvain