ALSA: Constify the snd_pcm_substream struct ops field

The ops field of the snd_pcm_substream struct is never modified inside the ALSA
core. Making it const allows drivers to declare their snd_pcm_ops struct as
const.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/pcm.h

@@ -384,7 +384,7 @@ struct snd_pcm_substream {
 	unsigned int dma_buf_id;
 	size_t dma_max;
 	/* -- hardware operations -- */
-	struct snd_pcm_ops *ops;
+	const struct snd_pcm_ops *ops;
 	/* -- runtime information -- */
 	struct snd_pcm_runtime *runtime;
         /* -- timer section -- */
@@ -871,7 +871,8 @@ const unsigned char *snd_pcm_format_silence_64(snd_pcm_format_t format);
 int snd_pcm_format_set_silence(snd_pcm_format_t format, void *buf, unsigned int frames);
 snd_pcm_format_t snd_pcm_build_linear_format(int width, int unsigned, int big_endian);
 
-void snd_pcm_set_ops(struct snd_pcm * pcm, int direction, struct snd_pcm_ops *ops);
+void snd_pcm_set_ops(struct snd_pcm * pcm, int direction,
+		     const struct snd_pcm_ops *ops);
 void snd_pcm_set_sync(struct snd_pcm_substream *substream);
 int snd_pcm_lib_interleave_len(struct snd_pcm_substream *substream);
 int snd_pcm_lib_ioctl(struct snd_pcm_substream *substream,

sound/core/pcm_lib.c

@@ -568,7 +568,8 @@ int snd_pcm_update_hw_ptr(struct snd_pcm_substream *substream)
  *
  * Sets the given PCM operators to the pcm instance.
  */
-void snd_pcm_set_ops(struct snd_pcm *pcm, int direction, struct snd_pcm_ops *ops)
+void snd_pcm_set_ops(struct snd_pcm *pcm, int direction,
+		     const struct snd_pcm_ops *ops)
 {
 	struct snd_pcm_str *stream = &pcm->streams[direction];
 	struct snd_pcm_substream *substream;