ASoC: fsl_ssi: Fix mode setting when changing channel number

This is a partial revert (in a cleaner way) of commit ebf08ae3
("ASoC: fsl_ssi: Keep ssi->i2s_net updated") to fix a regression
at test cases when switching between mono and stereo audio.

The problem is that ssi->i2s_net is initialized in set_dai_fmt()
only, while this set_dai_fmt() is only called during the dai-link
probe(). The original patch assumed set_dai_fmt() would be called
during every playback instance, so it failed at the overriding use
cases.

This patch adds the local variable i2s_net back to let regular use
cases still follow the mode settings from the set_dai_fmt().

Meanwhile, the original commit of keeping ssi->i2s_net updated was
to make set_tdm_slot() clean by checking the ssi->i2s_net directly
instead of reading SCR register. However, the change itself is not
necessary (or even harmful) because the set_tdm_slot() might fail
to check the slot number for Normal-Mode-None-Net settings while
mono audio cases still need 2 slots. So this patch can also fix it.
And it adds an extra line of comments to declare ssi->i2s_net does
not reflect the register value but merely the initial setting from
the set_dai_fmt().
Reported-by: Mika Penttilä <mika.penttila@nextfour.com>
Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Tested-by: Mika Penttilä <mika.penttila@nextfour.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/fsl/fsl_ssi.c

@@ -217,6 +217,7 @@ struct fsl_ssi_soc_data {
  * @dai_fmt: DAI configuration this device is currently used with
  * @streams: Mask of current active streams: BIT(TX) and BIT(RX)
  * @i2s_net: I2S and Network mode configurations of SCR register
+ *           (this is the initial settings based on the DAI format)
  * @synchronous: Use synchronous mode - both of TX and RX use STCK and SFCK
  * @use_dma: DMA is used or FIQ with stream filter
  * @use_dual_fifo: DMA with support for dual FIFO mode
@@ -829,16 +830,23 @@ static int fsl_ssi_hw_params(struct snd_pcm_substream *substream,
 	}
 
 	if (!fsl_ssi_is_ac97(ssi)) {
+		/*
+		 * Keep the ssi->i2s_net intact while having a local variable
+		 * to override settings for special use cases. Otherwise, the
+		 * ssi->i2s_net will lose the settings for regular use cases.
+		 */
+		u8 i2s_net = ssi->i2s_net;
+
 		/* Normal + Network mode to send 16-bit data in 32-bit frames */
 		if (fsl_ssi_is_i2s_cbm_cfs(ssi) && sample_size == 16)
-			ssi->i2s_net = SSI_SCR_I2S_MODE_NORMAL | SSI_SCR_NET;
+			i2s_net = SSI_SCR_I2S_MODE_NORMAL | SSI_SCR_NET;
 
 		/* Use Normal mode to send mono data at 1st slot of 2 slots */
 		if (channels == 1)
-			ssi->i2s_net = SSI_SCR_I2S_MODE_NORMAL;
+			i2s_net = SSI_SCR_I2S_MODE_NORMAL;
 
 		regmap_update_bits(regs, REG_SSI_SCR,
-				   SSI_SCR_I2S_NET_MASK, ssi->i2s_net);
+				   SSI_SCR_I2S_NET_MASK, i2s_net);
 	}
 
 	/* In synchronous mode, the SSI uses STCCR for capture */