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Commit 44dcbbb1 authored by Daniel Mack's avatar Daniel Mack Committed by Takashi Iwai
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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: default avatarDaniel Mack <zonque@gmail.com>
Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>
parent d24f5061

sound/usb/card.h

+1 −0
Original line number Diff line number Diff line
@@ -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;

sound/usb/pcm.c

+18 −1
Original line number Diff line number Diff line
@@ -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;



			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) {