[RFC PATCH 1/6] ALSA: compress: add Sample Rate Converter codec support

[Pierre-Louis Bossart]

On 8/6/24 12:26, Shengjiu Wang wrote:
> Add Sample Rate Converter(SRC) codec support, define the output
> format and rate for SRC.
> 
> Signed-off-by: Shengjiu Wang <shengjiu.wang at nxp.com>
> ---
>  include/uapi/sound/compress_offload.h | 2 ++
>  include/uapi/sound/compress_params.h  | 9 ++++++++-
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/sound/compress_offload.h b/include/uapi/sound/compress_offload.h
> index 98772b0cbcb7..8b2b72f94e26 100644
> --- a/include/uapi/sound/compress_offload.h
> +++ b/include/uapi/sound/compress_offload.h
> @@ -112,10 +112,12 @@ struct snd_compr_codec_caps {
>   * end of the track
>   * @SNDRV_COMPRESS_ENCODER_DELAY: no of samples inserted by the encoder at the
>   * beginning of the track
> + * @SNDRV_COMPRESS_SRC_RATIO_MOD: Resampling Ratio Modifier for sample rate converter
>   */
>  enum sndrv_compress_encoder {
>  	SNDRV_COMPRESS_ENCODER_PADDING = 1,
>  	SNDRV_COMPRESS_ENCODER_DELAY = 2,
> +	SNDRV_COMPRESS_SRC_RATIO_MOD = 3,
>  };

this sounds wrong to me. The sample rate converter is not an "encoder",
and the properties for padding/delay are totally specific to an encoder
function.

The other point is that I am not sure how standard this ratio_mod
parameter is. This could be totally specific to a specific
implementation, and another ASRC might have different parameters.

>  
>  /**
> diff --git a/include/uapi/sound/compress_params.h b/include/uapi/sound/compress_params.h
> index ddc77322d571..0843773ea6b4 100644
> --- a/include/uapi/sound/compress_params.h
> +++ b/include/uapi/sound/compress_params.h
> @@ -43,7 +43,8 @@
>  #define SND_AUDIOCODEC_BESPOKE               ((__u32) 0x0000000E)
>  #define SND_AUDIOCODEC_ALAC                  ((__u32) 0x0000000F)
>  #define SND_AUDIOCODEC_APE                   ((__u32) 0x00000010)
> -#define SND_AUDIOCODEC_MAX                   SND_AUDIOCODEC_APE
> +#define SND_AUDIOCODEC_SRC                   ((__u32) 0x00000011)
> +#define SND_AUDIOCODEC_MAX                   SND_AUDIOCODEC_SRC

I am not sure this is wise to change such definitions?
>  
>  /*
>   * Profile and modes are listed with bit masks. This allows for a
> @@ -324,6 +325,11 @@ struct snd_dec_ape {
>  	__u32 seek_table_present;
>  } __attribute__((packed, aligned(4)));
>  
> +struct snd_dec_src {
> +	__u32 format_out;
> +	__u32 rate_out;
> +} __attribute__((packed, aligned(4)));

Again I am not sure how standard those parameters are, and even if they
were if their representation is reusable.

And the compressed API has a good split between encoders and decoders, I
am not sure how an SRC can be classified as either.

>  union snd_codec_options {
>  	struct snd_enc_wma wma;
>  	struct snd_enc_vorbis vorbis;
> @@ -334,6 +340,7 @@ union snd_codec_options {
>  	struct snd_dec_wma wma_d;
>  	struct snd_dec_alac alac_d;
>  	struct snd_dec_ape ape_d;
> +	struct snd_dec_src src;
>  } __attribute__((packed, aligned(4)));
>  
>  /** struct snd_codec_desc - description of codec capabilities