[alsa-devel] [PATCH 2/2] ASoC: bcm2835: Add S16_LE support via packed DMA transfers

[Matthias Reichl]

The bcm2835-i2s driver already has support for the S16_LE format but
that format hasn't been made available because dmaengine_pcm didn't
support packed data transfers.

bcm2835-i2s needs 16-bit left+right channel data to be packed into
a 32-bit word, the FIFO register is 32-bit only and doesn't support
16-bit access.

Now that dmaengine_pcm supports packed transfers the format can
be made available by setting the SND_DMAENGINE_PCM_DAI_FLAG_PACK flag.

No further configuration is necessary:
- snd_dmaengine_dai_dma_data.addr_width is already set to
  DMA_SLAVE_BUSWIDTH_4_BYTES to force 32-bit DMA transfers
- dmaengine_pcm will pick up the S16_LE format from the DAI
  configuration and make it available since it's no longer
  masked out due to the PACK flag.
- there are no further corner cases to catch in hw_params,
  since the channel count is fixed at 2 we always have two
  16-bit stereo samples that can be transferred via 32-bit DMA

Signed-off-by: Matthias Reichl <hias at horus.com>
---
 sound/soc/bcm/bcm2835-i2s.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/sound/soc/bcm/bcm2835-i2s.c b/sound/soc/bcm/bcm2835-i2s.c
index 1c1f221..40ba01d 100644
--- a/sound/soc/bcm/bcm2835-i2s.c
+++ b/sound/soc/bcm/bcm2835-i2s.c
@@ -678,6 +678,15 @@ static int bcm2835_i2s_probe(struct platform_device *pdev)
 	dev->dma_data[SNDRV_PCM_STREAM_PLAYBACK].maxburst = 2;
 	dev->dma_data[SNDRV_PCM_STREAM_CAPTURE].maxburst = 2;
 
+	/*
+	 * Set the PACK flag to enable S16_LE support (2 S16_LE values
+	 * packed into 32-bit transfers).
+	 */
+	dev->dma_data[SNDRV_PCM_STREAM_PLAYBACK].flags =
+		SND_DMAENGINE_PCM_DAI_FLAG_PACK;
+	dev->dma_data[SNDRV_PCM_STREAM_CAPTURE].flags =
+		SND_DMAENGINE_PCM_DAI_FLAG_PACK;
+
 	/* BCLK ratio - use default */
 	dev->bclk_ratio = 0;
 
-- 
2.1.4