pcm: dmix: Allow disabling x86 optimizations

The dmix plugin has some optimized implementations for x86 using the
direct memory accesses, which was rather the original version, in
addition to the "generic" implementation using the semaphore
blocking.  The x86 implementation relies on the memory coherency *and*
the fast read/write on it.

For other architectures, this has been always disabled just because of
memory coherency.  But, the recent LPE audio development revealed
that, even on x86 platforms, the read/write performance might become
extremely bad when the buffer is marked as uncached.  Some drivers
already know the buffer is uncached, we need to switch to the generic
mode in such a case.

This patch introduces yet another flag to dmix configuration,
direct_memory_access, that indicates whether the x86-specific
optimization can be used or not.  Each driver can set the flag in its
cards config namespace, and the default dmix config refers to it.

As of this patch, only HDMI LPE Audio driver sets it.

Signed-off-by: Takashi Iwai <tiwai@suse.de>

src/conf/cards/HdmiLpeAudio.conf

@@ -20,6 +20,9 @@ HdmiLpeAudio.pcm.front.0 {
 	}
 }
 
+# uncached memory reads have a high penalty
+HdmiLpeAudio.dmix.direct_memory_access false
+
 # default with dmix+softvol
 HdmiLpeAudio.pcm.default {
 	@args [ CARD ]

src/conf/pcm/dmix.conf

@@ -49,6 +49,21 @@ pcm.!dmix {
 		@func refer
 		name defaults.pcm.ipc_perm
 	}
+	direct_memory_access {
+		@func refer
+		name {
+			@func concat
+			strings [
+				"cards."
+				{
+					@func card_driver
+					card $CARD
+				}
+				".dmix.direct_memory_access"
+			]
+		}
+		default true
+	}
 	slave {
 		pcm {
 			type hw

src/pcm/pcm_direct.c

@@ -1861,6 +1861,7 @@ int snd_pcm_direct_parse_open_conf(snd_config_t *root, snd_config_t *conf,
 	rec->slowptr = 1;
 	rec->max_periods = 0;
 	rec->var_periodsize = 1;
+	rec->direct_memory_access = 1;
 
 	/* read defaults */
 	if (snd_config_search(root, "defaults.pcm.dmix_max_periods", &n) >= 0) {
@@ -1974,6 +1975,13 @@ int snd_pcm_direct_parse_open_conf(snd_config_t *root, snd_config_t *conf,
 			rec->var_periodsize = err;
 			continue;
 		}
+		if (strcmp(id, "direct_memory_access") == 0) {
+			err = snd_config_get_bool(n);
+			if (err < 0)
+				return err;
+			rec->direct_memory_access = err;
+			continue;
+		}
 		SNDERR("Unknown field %s", id);
 		return -EINVAL;
 	}

src/pcm/pcm_direct.h

@@ -159,6 +159,7 @@ struct snd_pcm_direct {
 	unsigned int channels;		/* client's channels */
 	unsigned int *bindings;
 	unsigned int recoveries;	/* mirror of executed recoveries on slave */
+	int direct_memory_access;	/* use arch-optimized buffer RW */
 	union {
 		struct {
 			int shmid_sum;			/* IPC global sum ring buffer memory identification */
@@ -340,6 +341,7 @@ struct snd_pcm_direct_open_conf {
 	int slowptr;
 	int max_periods;
 	int var_periodsize;
+	int direct_memory_access;
 	snd_config_t *slave;
 	snd_config_t *bindings;
 };

src/pcm/pcm_dmix.c

@@ -1065,6 +1065,7 @@ int snd_pcm_dmix_open(snd_pcm_t **pcmp, const char *name,
 	dmix->max_periods = opts->max_periods;
 	dmix->var_periodsize = opts->var_periodsize;
 	dmix->sync_ptr = snd_pcm_dmix_sync_ptr;
+	dmix->direct_memory_access = opts->direct_memory_access;
 
  retry:
 	if (first_instance) {

src/pcm/pcm_dmix_i386.c

@@ -87,6 +87,11 @@ static void mix_select_callbacks(snd_pcm_direct_t *dmix)
 {
 	static int smp = 0, mmx = 0, cmov = 0;
 
+	if (!dmix->direct_memory_access) {
+		generic_mix_select_callbacks(dmix);
+		return;
+	}
+
 	if (!((1ULL<< dmix->shmptr->s.format) & i386_dmix_supported_format)) {
 		generic_mix_select_callbacks(dmix);
 		return;

src/pcm/pcm_dmix_x86_64.c

@@ -70,6 +70,11 @@ static void mix_select_callbacks(snd_pcm_direct_t *dmix)
 {
 	static int smp = 0;
 	
+	if (!dmix->direct_memory_access) {
+		generic_mix_select_callbacks(dmix);
+		return;
+	}
+
 	if (!((1ULL<< dmix->shmptr->s.format) & x86_64_dmix_supported_format)) {
 		generic_mix_select_callbacks(dmix);
 		return;