ASoC: Update snd_soc_dai_set_tdm_slot() documentation

There have been some conflicting interpretations of how
snd_soc_dai_set_tdm_slot() is supposed to work. This patch updates the
documentation to be more specific on the exact semantics to avoid such
problems in the future.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/soc-core.c

@@ -2119,15 +2119,27 @@ static int snd_soc_xlate_tdm_slot_mask(unsigned int slots,
 }
 
 /**
- * snd_soc_dai_set_tdm_slot - configure DAI TDM.
- * @dai: DAI
+ * snd_soc_dai_set_tdm_slot() - Configures a DAI for TDM operation
+ * @dai: The DAI to configure
  * @tx_mask: bitmask representing active TX slots.
  * @rx_mask: bitmask representing active RX slots.
  * @slots: Number of slots in use.
  * @slot_width: Width in bits for each slot.
  *
- * Configures a DAI for TDM operation. Both mask and slots are codec and DAI
- * specific.
+ * This function configures the specified DAI for TDM operation. @slot contains
+ * the total number of slots of the TDM stream and @slot_with the width of each
+ * slot in bit clock cycles. @tx_mask and @rx_mask are bitmasks specifying the
+ * active slots of the TDM stream for the specified DAI, i.e. which slots the
+ * DAI should write to or read from. If a bit is set the corresponding slot is
+ * active, if a bit is cleared the corresponding slot is inactive. Bit 0 maps to
+ * the first slot, bit 1 to the second slot and so on. The first active slot
+ * maps to the first channel of the DAI, the second active slot to the second
+ * channel and so on.
+ *
+ * TDM mode can be disabled by passing 0 for @slots. In this case @tx_mask,
+ * @rx_mask and @slot_width will be ignored.
+ *
+ * Returns 0 on success, a negative error code otherwise.
  */
 int snd_soc_dai_set_tdm_slot(struct snd_soc_dai *dai,
 	unsigned int tx_mask, unsigned int rx_mask, int slots, int slot_width)