[alsa-devel] snd_pcm_set_params faild on set_buffer_time_near and set_period_time_near

[Takashi Iwai]

On Tue, 21 Jul 2015 10:52:01 +0200,
Martin Geier wrote:
> 
> Hallo,
> 
> I tried to configure alsa device with snd_pcm_set_params function, 
> requested
> parameters was:
> channel 2, rate: 22050, soft_resample: 1, latency: 50000
> 
> Function snd_pcm_hw_params_set_buffer_time_near and 
> snd_pcm_hw_param_set_near
> is called with:
> min: 50000, max: 50000, mindir: 0, maxdir: -1
> 
> and input params was:
> 
> ACCESS:  RW_INTERLEAVED
> FORMAT:  S16_LE
> SUBFORMAT:  STD
> SAMPLE_BITS: 16
> FRAME_BITS: 32
> CHANNELS: 2
> RATE: 22050
> PERIOD_TIME: (362 743039)
> PERIOD_SIZE: [8 16384]
> PERIOD_BYTES: [32 65536]
> PERIODS: [2 19]
> BUFFER_TIME: (725 1486078)
> BUFFER_SIZE: [16 32768]
> BUFFER_BYTES: [64 131072]
> TICK_TIME: ALL
> 
> unfortunately, this function fails, on function snd_pcm_hw_param_set_first:
> 
> ALSA ERROR hw_params: set_near (BUFFER_TIME)
>             value = 50000 : Invalid argument
> ACCESS:  RW_INTERLEAVED
> FORMAT:  S16_LE
> SUBFORMAT:  STD
> SAMPLE_BITS: 16
> FRAME_BITS: 32
> CHANNELS: 2
> RATE: 22050
> PERIOD_TIME: (4580 5533)
> PERIOD_SIZE: NONE
> PERIOD_BYTES: [404 488]
> PERIODS: 10
> BUFFER_TIME: (50022 50023)
> BUFFER_SIZE: 1103
> BUFFER_BYTES: 4412
> TICK_TIME: ALL
> 
> This shouldn't be problem because in snd_pcm_set_params is
> snd_pcm_hw_params_set_period_time_near called.
> This function should be called with same params as 
> snd_pcm_hw_params_set_buffer_time_near
> but the input params are:
> 
> snd_pcm_hw_param_set_near: min: 12500, max: 12500, mindir: 0, maxdir: -1
> hw_params: snd_pcm_hw_param_set_near (PERIOD_TIME)
> ACCESS:  RW_INTERLEAVED
> FORMAT:  S16_LE
> SUBFORMAT:  STD
> SAMPLE_BITS: 16
> FRAME_BITS: 32
> CHANNELS: 2
> RATE: 22050
> PERIOD_TIME: (4580 5533)
> PERIOD_SIZE: NONE
> PERIOD_BYTES: [404 488]
> PERIODS: 10
> BUFFER_TIME: (50022 50023)
> BUFFER_SIZE: 1103
> BUFFER_BYTES: 4412
> TICK_TIME: ALL
> 
> because in snd_pcm_hw_param_set_near is
> 
> if (last)
>      err = snd_pcm_hw_param_set_last(pcm, params, var, val, dir);
> else
>      err = snd_pcm_hw_param_set_first(pcm, params, var, val, dir);
> if (err < 0)
>      dump_hw_params(params, "set_near", var, *val, err);
> return err;
> 
> and in error part is only debug output and not restore original parameters.
> 
> Environment:
> alsa-lib 1.0.27.1
> TI AM335x soc
> 
> Simple fix is attached, but I am not sure if it is correct.

Thanks for analysis and patch.  It's however rather a bug in
snd_pcm_set_params().  There is no guarantee to keep the old value in
snd_pcm_set_*(), thus the caller needs to take care of it instead.


Takashi