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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>
This commit is contained in:
Daniel Mack 2013-04-17 00:01:39 +08:00 committed by Takashi Iwai
parent d24f5061ee
commit 44dcbbb1cd
2 changed files with 19 additions and 1 deletions
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1
sound/usb/card.h
View File

@ -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;

19
sound/usb/pcm.c
View File

@ -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) {