ALSA: snd-usb: add support for DSD DOP stream transport
In order to provide a compatibility way for pushing DSD
samples through ordinary PCM channels, the "DoP open Standard" was
invented. See http://www.dsd-guide.com for the official document.
The host is required to stuff DSD marker bytes (0x05, 0xfa,
alternating) in the MSB of 24 bit wide samples on the bus, in addition
to the 16 bits of actual DSD sample payload.
To support this, the hardware and software stride logic in the driver
has to be tweaked a bit, as we make the userspace believe we're
operating on 16 bit samples, while we in fact push one more byte per
channel down to the hardware.
The DOP runtime information is stored in struct snd_usb_substream, so
we can keep track of our state across multiple calls to
prepare_playback_urb_dsd_dop().
Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
index 7e9c55a..32d0b41 100644
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -578,6 +578,15 @@
int is_playback = usb_pipeout(ep->pipe);
int frame_bits = snd_pcm_format_physical_width(pcm_format) * channels;
+ if (pcm_format == SNDRV_PCM_FORMAT_DSD_U16_LE && fmt->dsd_dop) {
+ /*
+ * When operating in DSD DOP mode, the size of a sample frame
+ * in hardware differs from the actual physical format width
+ * because we need to make room for the DOP markers.
+ */
+ frame_bits += channels << 3;
+ }
+
ep->datainterval = fmt->datainterval;
ep->stride = frame_bits >> 3;
ep->silence_value = pcm_format == SNDRV_PCM_FORMAT_U8 ? 0x80 : 0;