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
