ALSA: pcm: Add an ioctl to specify the supported protocol version

We have an ioctl to inform the PCM protocol version the running kernel
supports, but there is no way to know which protocol version the
user-space can understand. This lack of information caused headaches
in the past when we tried to extend the ABI. For example, because we
couldn't guarantee the validity of the reserved bytes, we had to
introduce a new ioctl SNDRV_PCM_IOCTL_STATUS_EXT for assigning a few
new fields in the formerly reserved bits. If we could know that it's
a new alsa-lib, we could assume the availability of the new fields,
thus we could have reused the existing SNDRV_PCM_IOCTL_STATUS.

In order to improve the ABI extensibility, this patch adds a new ioctl
for user-space to inform its supporting protocol version to the
kernel. By reporting the supported protocol from user-space, the
kernel can judge which feature should be provided and which not.

With the addition of the new ioctl, the PCM protocol version is bumped
to 2.0.14, too. User-space checks the kernel protocol version via
SNDRV_PCM_INFO_PVERSION, then it sets the supported version back via
SNDRV_PCM_INFO_USER_PVERSION.

Reviewed-by: Takashi Sakamoto <[e-mail]>
Signed-off-by: Takashi Iwai <[e-mail]>

include/sound/pcm.h

···
 struct snd_pcm_file {
 	struct snd_pcm_substream *substream;
 	int no_compat_mmap;
+	unsigned int user_pversion;	/* supported protocol version */
 };
 
 struct snd_pcm_hw_rule;

include/uapi/sound/asound.h

···
  *                                                                           *
  *****************************************************************************/
 
-#define SNDRV_PCM_VERSION		SNDRV_PROTOCOL_VERSION(2, 0, 13)
+#define SNDRV_PCM_VERSION		SNDRV_PROTOCOL_VERSION(2, 0, 14)
 
 typedef unsigned long snd_pcm_uframes_t;
 typedef signed long snd_pcm_sframes_t;
···
 #define SNDRV_PCM_IOCTL_INFO		_IOR('A', 0x01, struct snd_pcm_info)
 #define SNDRV_PCM_IOCTL_TSTAMP		_IOW('A', 0x02, int)
 #define SNDRV_PCM_IOCTL_TTSTAMP		_IOW('A', 0x03, int)
+#define SNDRV_PCM_IOCTL_USER_PVERSION	_IOW('A', 0x04, int)
 #define SNDRV_PCM_IOCTL_HW_REFINE	_IOWR('A', 0x10, struct snd_pcm_hw_params)
 #define SNDRV_PCM_IOCTL_HW_PARAMS	_IOWR('A', 0x11, struct snd_pcm_hw_params)
 #define SNDRV_PCM_IOCTL_HW_FREE		_IO('A', 0x12)

sound/core/pcm_compat.c

···
 	case SNDRV_PCM_IOCTL_INFO:
 	case SNDRV_PCM_IOCTL_TSTAMP:
 	case SNDRV_PCM_IOCTL_TTSTAMP:
+	case SNDRV_PCM_IOCTL_USER_PVERSION:
 	case SNDRV_PCM_IOCTL_HWSYNC:
 	case SNDRV_PCM_IOCTL_PREPARE:
 	case SNDRV_PCM_IOCTL_RESET:

sound/core/pcm_native.c

···
 				 struct snd_pcm_substream *substream,
 				 unsigned int cmd, void __user *arg)
 {
+	struct snd_pcm_file *pcm_file = file->private_data;
+
 	switch (cmd) {
 	case SNDRV_PCM_IOCTL_PVERSION:
 		return put_user(SNDRV_PCM_VERSION, (int __user *)arg) ? -EFAULT : 0;
···
 		return 0;
 	case SNDRV_PCM_IOCTL_TTSTAMP:
 		return snd_pcm_tstamp(substream, arg);
+	case SNDRV_PCM_IOCTL_USER_PVERSION:
+		if (get_user(pcm_file->user_pversion,
+			     (unsigned int __user *)arg))
+			return -EFAULT;
+		return 0;
 	case SNDRV_PCM_IOCTL_HW_REFINE:
 		return snd_pcm_hw_refine_user(substream, arg);
 	case SNDRV_PCM_IOCTL_HW_PARAMS: