[alsa-devel] [RFC PATCH 1/4] alsa: make hw_params negotiation infrastructure 'bclk_ratio aware'

[Russell King - ARM Linux admin]

On Tue, Mar 05, 2019 at 01:42:32PM +0900, Takashi Sakamoto wrote:
> Hi,
> 
> On Mon, Mar 04, 2019 at 11:59:52AM -0500, Sven Van Asbroeck wrote:
> > Negotiation seems to work ok, but bclk_ratio is exposed to
> > userspace via snd_pcm_hw_params, which is not acceptable.
> > 
> > Constrain bclk_ratio by:
> > - cpu   dai capabilities && rules
> > - codec dai capabilities && rules
> > - minimum bclk_ratio is sample_width * channels
> > 
> > In hw_params_choose(), pick the smallest supported bclk_ratio,
> > which should correspond to the most efficient solution.
> > 
> > If cpu and codec dais do not specify or constrain supported
> > bclk_ratios, alsa will pick sample_width * channels.
> > 
> > Signed-off-by: Sven Van Asbroeck <TheSven73 at gmail.com>
> > ---
> >  include/sound/pcm.h         | 11 +++++++++++
> >  include/sound/soc.h         |  2 ++
> >  include/uapi/sound/asound.h |  5 +++--
> >  sound/core/pcm_native.c     | 34 +++++++++++++++++++++++++++++++++-
> >  sound/soc/soc-pcm.c         |  8 ++++++++
> >  5 files changed, 57 insertions(+), 3 deletions(-)
> 
> In UAPI of Linux sound subsystem, sample formats are represented
> in enumerators prefixed with 'SNDRV_PCM_FORMAT_'. In the enumerators,
> sample bits and padding bits are represented:
> 
>               S8 S16 S20 S24 S32 S18_3 S20_3 S24_3
> sample-bits:   8  16  20  24  32  18    20    24 (=width)
> padding-bits:  0   0  12   8   0   6    12     0
> total bits:    8  16  32  32  32  24    24    24 (=physical_width)
>  
> When indicating a certain bit ratio of BCLK/WS from userspace,
> applications use correct combination of the above format (=physical_width)
> and the number of samples per audio data frame (=channels).

You seem to be conflating the in-memory format with the on-wire format.
These are two entirely different things.  Your table above clearly shows
the in-memory format, not the on-wire format.

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