ALSA: snd-usb: add support for bit-reversed byte formats

There is quite some confusion around the bit-ordering in DSD samples,
and no general agreement that defines whether hardware is supposed to
expect the oldest sample in the MSB or the LSB of a byte.

ALSA will hence set the rule that on the software API layer, bytes
always carry the oldest bit in the most significant bit of a byte, and
the driver has to translate that at runtime in order to match the
hardware layout.

This patch adds support for this by adding a boolean flag to the
audio format struct.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

2 files changed, 19 insertions, 1 deletions

diff --git a/sound/usb/card.h b/sound/usb/card.h
index ac55477ce6dd..bf2889a2cae5 100644
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -29,6 +29,7 @@ struct audioformat {
         unsigned char clock;            /* associated clock */
         struct snd_pcm_chmap_elem *chmap; /* (optional) channel map */
         bool dsd_dop;                   /* add DOP headers in case of DSD samples */
+        bool dsd_bitrev;                /* reverse the bits of each DSD sample */
 };
 
 struct snd_usb_substream;
diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index 4cd917cf058e..9723f3ceb155 100644
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -16,6 +16,7 @@
 
 #include <linux/init.h>
 #include <linux/slab.h>
+#include <linux/bitrev.h>
 #include <linux/ratelimit.h>
 #include <linux/usb.h>
 #include <linux/usb/audio.h>
@@ -1264,7 +1265,12 @@ static inline void fill_playback_urb_dsd_dop(struct snd_usb_substream *subs,
                 } else {
                         /* stuff the DSD payload */
                         int idx = (src_idx + subs->dsd_dop.byte_idx - 1) % wrap;
-                        dst[dst_idx++] = src[idx];
+
+                        if (subs->cur_audiofmt->dsd_bitrev)
+                                dst[dst_idx++] = bitrev8(src[idx]);
+                        else
+                                dst[dst_idx++] = src[idx];
+
                         subs->hwptr_done++;
                 }
         }
@@ -1330,6 +1336,17 @@ static void prepare_playback_urb(struct snd_usb_substream *subs,
         if (unlikely(subs->pcm_format == SNDRV_PCM_FORMAT_DSD_U16_LE &&
                      subs->cur_audiofmt->dsd_dop)) {
                 fill_playback_urb_dsd_dop(subs, urb, bytes);
+        } else if (unlikely(subs->pcm_format == SNDRV_PCM_FORMAT_DSD_U8 &&
+                           subs->cur_audiofmt->dsd_bitrev)) {
+                /* bit-reverse the bytes */
+                u8 *buf = urb->transfer_buffer;
+                for (i = 0; i < bytes; i++) {
+                        int idx = (subs->hwptr_done + i)
+                                % (runtime->buffer_size * stride);
+                        buf[i] = bitrev8(runtime->dma_area[idx]);
+                }
+
+                subs->hwptr_done += bytes;
         } else {
                 /* usual PCM */
                 if (subs->hwptr_done + bytes > runtime->buffer_size * stride) {