ALSA: pcm: Add SNDRV_PCM_INFO_EXPLICIT_SYNC flag

ALSA PCM core has an optimized way to communicate with user-space for
its control and status data via mmap on the supported architectures
like x86.  Depending on the situation, however, we'd rather want to
enforce user-space notifying the applptr or hwptr change explicitly
via ioctl.  For example, the upcoming non-contig and non-coherent
buffer handling would need an explicit sync, and this needs to catch
the applptr and hwptr changes.  Also, ASoC SOF driver will have the
SPIB support that has the similar requirement for the explicit control
of the applptr and hwptr.

This patch adds the new PCM hardware info flag,
SNDRV_PCM_INFO_EXPLICIT_SYNC.  When this flag is set, PCM core
disables both the control and the status mmap, which enforces
user-space to update via SYNC_PTR ioctl.  In that way, drivers can
catch the applptr and hwptr update and apply the sync operation if
needed.

Link: https://lore.kernel.org/r/20210812113818.6479-1-tiwai@suse.de
Link: https://lore.kernel.org/r/20210610205326.1176400-1-pierre-louis.bossart@linux.intel.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/uapi/sound/asound.h

@@ -299,6 +299,7 @@ typedef int __bitwise snd_pcm_subformat_t;
 #define SNDRV_PCM_INFO_HAS_LINK_ABSOLUTE_ATIME     0x02000000  /* report absolute hardware link audio time, not reset on startup */
 #define SNDRV_PCM_INFO_HAS_LINK_ESTIMATED_ATIME    0x04000000  /* report estimated link audio time */
 #define SNDRV_PCM_INFO_HAS_LINK_SYNCHRONIZED_ATIME 0x08000000  /* report synchronized audio/system time */
+#define SNDRV_PCM_INFO_EXPLICIT_SYNC	0x10000000	/* needs explicit sync of pointers and data */
 
 #define SNDRV_PCM_INFO_DRAIN_TRIGGER	0x40000000		/* internal kernel flag - trigger in drain */
 #define SNDRV_PCM_INFO_FIFO_IN_FRAMES	0x80000000	/* internal kernel flag - FIFO size is in frames */

sound/core/pcm_native.c

@@ -3613,6 +3613,12 @@ static int snd_pcm_mmap_control(struct snd_pcm_substream *substream, struct file
 
 static bool pcm_status_mmap_allowed(struct snd_pcm_file *pcm_file)
 {
+	/* If drivers require the explicit sync (typically for non-coherent
+	 * pages), we have to disable the mmap of status and control data
+	 * to enforce the control via SYNC_PTR ioctl.
+	 */
+	if (pcm_file->substream->runtime->hw.info & SNDRV_PCM_INFO_EXPLICIT_SYNC)
+		return false;
 	/* See pcm_control_mmap_allowed() below.
 	 * Since older alsa-lib requires both status and control mmaps to be
 	 * coupled, we have to disable the status mmap for old alsa-lib, too.
@@ -3627,6 +3633,9 @@ static bool pcm_control_mmap_allowed(struct snd_pcm_file *pcm_file)
 {
 	if (pcm_file->no_compat_mmap)
 		return false;
+	/* see above */
+	if (pcm_file->substream->runtime->hw.info & SNDRV_PCM_INFO_EXPLICIT_SYNC)
+		return false;
 	/* Disallow the control mmap when SYNC_APPLPTR flag is set;
 	 * it enforces the user-space to fall back to snd_pcm_sync_ptr(),
 	 * thus it effectively assures the manual update of appl_ptr.