[PATCH RFC v2 2/4] ALSA: pcm: Add SNDRV_PCM_INFO_EXPLICIT_SYNC flag

[Takashi Iwai <tiwai@suse.de>]

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.

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.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 include/uapi/sound/asound.h | 1 +
 sound/core/pcm_native.c     | 9 +++++++++
 2 files changed, 10 insertions(+)

diff --git a/include/uapi/sound/asound.h b/include/uapi/sound/asound.h
index d17c061950df..1d84ec9db93b 100644
--- a/include/uapi/sound/asound.h
+++ b/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 */
diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index dc9fa312fadd..d233cb3b41d8 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -3621,6 +3621,12 @@ 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.
@@ -3635,6 +3641,9 @@ 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.
-- 
2.26.2