1 files changed, 32 insertions, 0 deletions

diff --git a/include/sound/control.h b/include/sound/control.h
index 4cf8f7aaa13f..ef96f07aa03b 100644
--- a/include/sound/control.h
+++ b/include/sound/control.h
@@ -176,12 +176,44 @@ int _snd_ctl_add_slave(struct snd_kcontrol *master, struct snd_kcontrol *slave,
 /* optional flags for slave */
 #define SND_CTL_SLAVE_NEED_UPDATE       (1 << 0)
 
+/**
+ * snd_ctl_add_slave - Add a virtual slave control
+ * @master: vmaster element
+ * @slave: slave element to add
+ *
+ * Add a virtual slave control to the given master element created via
+ * snd_ctl_create_virtual_master() beforehand.
+ * Returns zero if successful or a negative error code.
+ *
+ * All slaves must be the same type (returning the same information
+ * via info callback).  The fucntion doesn't check it, so it's your
+ * responsibility.
+ *
+ * Also, some additional limitations:
+ * at most two channels,
+ * logarithmic volume control (dB level) thus no linear volume,
+ * master can only attenuate the volume without gain
+ */
 static inline int
 snd_ctl_add_slave(struct snd_kcontrol *master, struct snd_kcontrol *slave)
 {
         return _snd_ctl_add_slave(master, slave, 0);
 }
 
+/**
+ * snd_ctl_add_slave_uncached - Add a virtual slave control
+ * @master: vmaster element
+ * @slave: slave element to add
+ *
+ * Add a virtual slave control to the given master.
+ * Unlike snd_ctl_add_slave(), the element added via this function
+ * is supposed to have volatile values, and get callback is called
+ * at each time quried from the master.
+ *
+ * When the control peeks the hardware values directly and the value
+ * can be changed by other means than the put callback of the element,
+ * this function should be used to keep the value always up-to-date.
+ */
 static inline int
 snd_ctl_add_slave_uncached(struct snd_kcontrol *master,
                            struct snd_kcontrol *slave)