ALSA: pcm: clarify and fix default msbits value for all formats

Return used most significant bits from sample bit-width rather than the whole
physical sample word size. The starting bit offset is defined in the format
itself.

The behaviour is not changed for 32-bit formats like S32_LE. But with this
change - msbits value 24 instead 32 is returned for 24-bit formats like S24_LE
etc.

Also, commit 2112aa03 ("ALSA: pcm: Introduce MSBITS subformat interface")
compares sample bit-width not physical sample bit-width to reset MSBITS_MAX bit
from the subformat bitmask.

Probably no applications are using msbits value for other than S32_LE/U32_LE
formats, because no drivers are reducing msbits value for other formats (with
the msb offset) at the moment.

For sanity, increase PCM protocol version, letting the user space to detect
the changed behaviour.
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20240222173649.1447549-1-perex@perex.czSigned-off-by: Takashi Iwai <tiwai@suse.de>

include/uapi/sound/asound.h

@@ -142,7 +142,7 @@ struct snd_hwdep_dsp_image {
  *                                                                           *
  *****************************************************************************/
 
-#define SNDRV_PCM_VERSION		SNDRV_PROTOCOL_VERSION(2, 0, 16)
+#define SNDRV_PCM_VERSION		SNDRV_PROTOCOL_VERSION(2, 0, 17)
 
 typedef unsigned long snd_pcm_uframes_t;
 typedef signed long snd_pcm_sframes_t;
@@ -416,7 +416,7 @@ struct snd_pcm_hw_params {
 	unsigned int rmask;		/* W: requested masks */
 	unsigned int cmask;		/* R: changed masks */
 	unsigned int info;		/* R: Info flags for returned setup */
-	unsigned int msbits;		/* R: used most significant bits */
+	unsigned int msbits;		/* R: used most significant bits (in sample bit-width) */
 	unsigned int rate_num;		/* R: rate numerator */
 	unsigned int rate_den;		/* R: rate denominator */
 	snd_pcm_uframes_t fifo_size;	/* R: chip FIFO size in frames */

sound/core/pcm_native.c

@@ -486,6 +486,11 @@ static int fixup_unreferenced_params(struct snd_pcm_substream *substream,
 		i = hw_param_interval_c(params, SNDRV_PCM_HW_PARAM_SAMPLE_BITS);
 		if (snd_interval_single(i))
 			params->msbits = snd_interval_value(i);
+		m = hw_param_mask_c(params, SNDRV_PCM_HW_PARAM_FORMAT);
+		if (snd_mask_single(m)) {
+			snd_pcm_format_t format = (__force snd_pcm_format_t)snd_mask_min(m);
+			params->msbits = snd_pcm_format_width(format);
+		}
 	}
 
 	if (params->msbits) {