ALSA: bcm2835: add memory-mapped I/O mode for audio stream

ALSA supports two transfers methods for PCM playback: Read/Write
transfer where samples are writtern to the device using standard
read and write functions and Direct Read/Write transfers where
samples can be written directly to a mapped memory area and the
driver is signaled once this has been done.

The bcm2835 driver only supported Read/Write transfer method so
this patch adds mmap support to the driver.

The ARM CPU is not able to directly address the audio device
hardware buffer so audio samples are sent to the device using
a message passing interface (vchiq).

Since hardware buffers can't be directly mapped to user-space
memory, an intermediate buffer (using the PCM indirect API) is
needed to store the audio samples and push them to the device
through videocore.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>

Author: Javier Martinez Canillas

sound/arm/bcm2835-pcm.c

@@ -19,7 +19,8 @@
 
 /* hardware definition */
 static struct snd_pcm_hardware snd_bcm2835_playback_hw = {
-	.info = (SNDRV_PCM_INFO_INTERLEAVED | SNDRV_PCM_INFO_BLOCK_TRANSFER),
+	.info = (SNDRV_PCM_INFO_INTERLEAVED | SNDRV_PCM_INFO_BLOCK_TRANSFER |
+		 SNDRV_PCM_INFO_MMAP | SNDRV_PCM_INFO_MMAP_VALID),
 	.formats = SNDRV_PCM_FMTBIT_U8 | SNDRV_PCM_FMTBIT_S16_LE,
 	.rates = SNDRV_PCM_RATE_CONTINUOUS | SNDRV_PCM_RATE_8000_48000,
 	.rate_min = 8000,
@@ -251,6 +252,12 @@ static int snd_bcm2835_pcm_prepare(struct snd_pcm_substream *substream)
 
 	audio_info(" .. IN\n");
 
+	memset(&alsa_stream->pcm_indirect, 0, sizeof(alsa_stream->pcm_indirect));
+
+	alsa_stream->pcm_indirect.hw_buffer_size =
+	alsa_stream->pcm_indirect.sw_buffer_size =
+		snd_pcm_lib_buffer_bytes(substream);
+
 	alsa_stream->buffer_size = snd_pcm_lib_buffer_bytes(substream);
 	alsa_stream->period_size = snd_pcm_lib_period_bytes(substream);
 	alsa_stream->pos = 0;
@@ -263,6 +270,32 @@ static int snd_bcm2835_pcm_prepare(struct snd_pcm_substream *substream)
 	return 0;
 }
 
+static void snd_bcm2835_pcm_transfer(struct snd_pcm_substream *substream,
+				    struct snd_pcm_indirect *rec, size_t bytes)
+{
+	struct snd_pcm_runtime *runtime = substream->runtime;
+	bcm2835_alsa_stream_t *alsa_stream = runtime->private_data;
+	void *src = (void *)(substream->runtime->dma_area + rec->sw_data);
+	int err;
+
+	err = bcm2835_audio_write(alsa_stream, bytes, src);
+	if (err)
+		audio_error(" Failed to transfer to alsa device (%d)\n", err);
+
+}
+
+static int snd_bcm2835_pcm_ack(struct snd_pcm_substream *substream)
+{
+	struct snd_pcm_runtime *runtime = substream->runtime;
+	bcm2835_alsa_stream_t *alsa_stream = runtime->private_data;
+	struct snd_pcm_indirect *pcm_indirect = &alsa_stream->pcm_indirect;
+
+	pcm_indirect->hw_queue_size = runtime->hw.buffer_bytes_max;
+	snd_pcm_indirect_playback_transfer(substream, pcm_indirect,
+					   snd_bcm2835_pcm_transfer);
+	return 0;
+}
+
 /* trigger callback */
 static int snd_bcm2835_pcm_trigger(struct snd_pcm_substream *substream, int cmd)
 {
@@ -279,6 +312,11 @@ static int snd_bcm2835_pcm_trigger(struct snd_pcm_substream *substream, int cmd)
 		if (!alsa_stream->running) {
 			err = bcm2835_audio_start(alsa_stream);
 			if (err == 0) {
+				alsa_stream->pcm_indirect.hw_io =
+				alsa_stream->pcm_indirect.hw_data =
+					bytes_to_frames(runtime,
+							alsa_stream->pos);
+				substream->ops->ack(substream);
 				alsa_stream->running = 1;
 				alsa_stream->draining = 1;
 			} else {
@@ -327,30 +365,9 @@ snd_bcm2835_pcm_pointer(struct snd_pcm_substream *substream)
 		      alsa_stream->pos);
 
 	audio_info(" .. OUT\n");
-	return bytes_to_frames(runtime, alsa_stream->pos);
-}
-
-static int snd_bcm2835_pcm_copy(struct snd_pcm_substream *substream,
-				int channel, snd_pcm_uframes_t pos, void *src,
-				snd_pcm_uframes_t count)
-{
-	int ret;
-	struct snd_pcm_runtime *runtime = substream->runtime;
-	bcm2835_alsa_stream_t *alsa_stream = runtime->private_data;
-
-	audio_info(" .. IN\n");
-	audio_debug("copy.......... (%d) hwptr=%d appl=%d pos=%d\n",
-		      frames_to_bytes(runtime, count), frames_to_bytes(runtime,
-								       runtime->
-								       status->
-								       hw_ptr),
-		      frames_to_bytes(runtime, runtime->control->appl_ptr),
-		      alsa_stream->pos);
-	ret =
-	    bcm2835_audio_write(alsa_stream, frames_to_bytes(runtime, count),
-				src);
-	audio_info(" .. OUT\n");
-	return ret;
+	return snd_pcm_indirect_playback_pointer(substream,
+						 &alsa_stream->pcm_indirect,
+						 alsa_stream->pos);
 }
 
 static int snd_bcm2835_pcm_lib_ioctl(struct snd_pcm_substream *substream,
@@ -372,7 +389,7 @@ static struct snd_pcm_ops snd_bcm2835_playback_ops = {
 	.prepare = snd_bcm2835_pcm_prepare,
 	.trigger = snd_bcm2835_pcm_trigger,
 	.pointer = snd_bcm2835_pcm_pointer,
-	.copy = snd_bcm2835_pcm_copy,
+	.ack = snd_bcm2835_pcm_ack,
 };
 
 /* create a pcm device */

sound/arm/bcm2835.h

@@ -23,6 +23,7 @@
 #include <sound/initval.h>
 #include <sound/pcm.h>
 #include <sound/pcm_params.h>
+#include <sound/pcm-indirect.h>
 #include <linux/workqueue.h>
 
 /*
@@ -110,6 +111,7 @@ typedef struct bcm2835_chip {
 typedef struct bcm2835_alsa_stream {
 	bcm2835_chip_t *chip;
 	struct snd_pcm_substream *substream;
+	struct snd_pcm_indirect pcm_indirect;
 
 	struct semaphore buffers_update_sem;
 	struct semaphore control_sem;