Re: [PATCH 1/2] ALSA: pcm: add SNDRV_PCM_FORMAT_{S, U}20_4

[Takashi Sakamoto <o-takashi@sakamocchi.jp>]

On Nov 23 2017 04:17, Maciej S. Szmigiero wrote:
> This format is similar to existing SNDRV_PCM_FORMAT_{S,U}20_3 that keep
> 20-bit PCM samples in 3 bytes, however i.MX6 platform SSI FIFO does not
> allow 3-byte accesses (including DMA) so a 4-byte format is needed for it.
> 
> Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
> ---
>   include/sound/pcm.h         |  8 ++++++++
>   include/sound/soc-dai.h     |  2 ++
>   include/uapi/sound/asound.h | 10 +++++++++-
>   sound/core/pcm_misc.c       | 16 ++++++++++++++++
>   4 files changed, 35 insertions(+), 1 deletion(-)
>
> ... >
> diff --git a/include/uapi/sound/asound.h b/include/uapi/sound/asound.h
> index c227ccba60ae..69b661816491 100644
> --- a/include/uapi/sound/asound.h
> +++ b/include/uapi/sound/asound.h
> @@ -236,7 +236,11 @@ typedef int __bitwise snd_pcm_format_t;
>   #define	SNDRV_PCM_FORMAT_DSD_U32_LE	((__force snd_pcm_format_t) 50) /* DSD, 4-byte samples DSD (x32), little endian */
>   #define	SNDRV_PCM_FORMAT_DSD_U16_BE	((__force snd_pcm_format_t) 51) /* DSD, 2-byte samples DSD (x16), big endian */
>   #define	SNDRV_PCM_FORMAT_DSD_U32_BE	((__force snd_pcm_format_t) 52) /* DSD, 4-byte samples DSD (x32), big endian */
> -#define	SNDRV_PCM_FORMAT_LAST		SNDRV_PCM_FORMAT_DSD_U32_BE
> +#define	SNDRV_PCM_FORMAT_S20_4LE	((__force snd_pcm_format_t) 53)	/* in four bytes */
> +#define	SNDRV_PCM_FORMAT_S20_4BE	((__force snd_pcm_format_t) 54)	/* in four bytes */
> +#define	SNDRV_PCM_FORMAT_U20_4LE	((__force snd_pcm_format_t) 55)	/* in four bytes */
> +#define	SNDRV_PCM_FORMAT_U20_4BE	((__force snd_pcm_format_t) 56)	/* in four bytes */
> +#define	SNDRV_PCM_FORMAT_LAST		SNDRV_PCM_FORMAT_U20_4BE

In my opinion, for this type of definition, it's better to declare 
left/right-adjusted or padding side. (Of course, silence definition is 
already a hint, however the lack of information forces developers to 
have a careful behaviour to handle entries on the list.)

(I note that in current ALSA PCM interface there's no way to deliver 
MSB/LSB-first information about sample format.)

Additionally, alsa-lib includes some codes related to the definition[1]. 
If you'd like to thing goes well out of ALSA SoC part, it's better to 
submit changes to the library as well.

[1] 
http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=src/pcm/pcm_misc.c;h=5420b1895713a3aec3624a5218794a7b49baf167;hb=HEAD


Regards

Takashi Sakamoto