Skip to content

Commit

Permalink
ALSA: snd-usb: add support for bit-reversed byte formats
Browse files Browse the repository at this point in the history
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>
  • Loading branch information
Daniel Mack authored and Takashi Iwai committed Apr 18, 2013
1 parent d24f506 commit 44dcbbb
Show file tree
Hide file tree
Showing 2 changed files with 19 additions and 1 deletion.
1 change: 1 addition & 0 deletions sound/usb/card.h
Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand Down
19 changes: 18 additions & 1 deletion sound/usb/pcm.c
Original file line number Diff line number Diff line change
Expand Up @@ -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>
Expand Down Expand Up @@ -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++;
}
}
Expand Down Expand Up @@ -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) {
Expand Down

0 comments on commit 44dcbbb

Please sign in to comment.