ALSA: pcm: introduce INFO_NO_REWINDS flag

When the hardware can only deal with a monotonically increasing
appl_ptr, this flag can be set.

In case the application requests a rewind, be it with a
snd_pcm_rewind() or with a direct change of a mmap'ed pointer followed
by a SNDRV_PCM_IOCTL_SYNC_PTR, this patch checks if a rewind
occurred and returns an error.

Credits to Takashi Iwai for identifying the path with SYNC_PTR and
suggesting the pointer checks.
Suggested-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20211119230852.206310-3-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

include/uapi/sound/asound.h

@@ -300,7 +300,7 @@ typedef int __bitwise snd_pcm_subformat_t;
 #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_NO_REWINDS	0x20000000	/* hardware can only support monotonic changes of appl_ptr */
 #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_lib.c

@@ -2128,6 +2128,7 @@ int pcm_lib_apply_appl_ptr(struct snd_pcm_substream *substream,
 {
 	struct snd_pcm_runtime *runtime = substream->runtime;
 	snd_pcm_uframes_t old_appl_ptr = runtime->control->appl_ptr;
+	snd_pcm_sframes_t diff;
 	int ret;
 
 	if (old_appl_ptr == appl_ptr)
@@ -2135,6 +2136,19 @@ int pcm_lib_apply_appl_ptr(struct snd_pcm_substream *substream,
 
 	if (appl_ptr >= runtime->boundary)
 		return -EINVAL;
+	/*
+	 * check if a rewind is requested by the application
+	 */
+	if (substream->runtime->info & SNDRV_PCM_INFO_NO_REWINDS) {
+		diff = appl_ptr - old_appl_ptr;
+		if (diff >= 0) {
+			if (diff > runtime->buffer_size)
+				return -EINVAL;
+		} else {
+			if (runtime->boundary + diff > runtime->buffer_size)
+				return -EINVAL;
+		}
+	}
 
 	runtime->control->appl_ptr = appl_ptr;
 	if (substream->ops->ack) {