ASoC: tas2770: Support setting the PDM TX slot

We don't actually support configuring the PDM input right now. Rather,
this is useful as a hack.

On Apple Silicon machines, amps are split between two I2S buses which
are logically ANDed internally at the SoC. Odd and even slot groups are
driven by amps on either bus respectively. Since the signals are ANDed,
unused slot groups must be driven as zero to avoid corrupting the data
from the other side.

On most recent machines (TAS2764-based), this is accomplished using the
"SDOUT zero mask" feature of that chip. Unfortunately, TAS2770 does not
support this. It does support zeroing out *all* unused slots, which
works well for machines with a single amp per I2S bus. That is all,
except one.

The 13" M1 MacBook Pro is the only machine using TAS2764 and two amps
per I2S bus:

L Bus: SPK0I SPK0V Hi-Z  Hi-Z  SPK2I SPK2V Hi-Z  Hi-Z
R Bus: Hi-Z  Hi-Z  SPK1I SPK2V Hi-Z  Hi-Z  SPK3I SPK3V

To ensure uncorrupted data, we need to force all the Hi-Z periods to
zero. We cannot use the "force all zero" feature, as that would cause a
bus conflict between both amps. We can use the pull-down feature, but
that leaves a few bits of garbage on the trailing edge of the speaker
data, since the pull-down is weak.

This is where the PDM transmit feature comes in. With PDM grounded and
disabled (the default state), the PDM slot is transmitted as all zeroes.
We can use that to force a zero 16-bit slot after the voltage data for
each speaker, cleaning it up. Then the pull-down ensures the line stays
low for the subsequent slot:

L Bus: SPK0I SPK0V PDM0  PulDn SPK2I SPK2V PDM0  PulDn
R Bus: PDM0  PulDn SPK1I SPK2V PDM0  PulDn SPK3I SPK3V

Yes, this is a horrible hack, but it beats adding dummy slots that would
be visible to the userspace capture side. There may be some other way to
fix the logical AND behavior on the MCA side... that would make this
unnecessary.

("How does Apple deal with this"? - they don't, macOS does not use
IVSENSE on TAS2764 machines even though it's physically wired up,
but we want to do so on Linux.)

Signed-off-by: Hector Martin <marcan@marcan.st>

Author: marcan

sound/soc/codecs/tas2770.c

@@ -251,6 +251,19 @@ static int tas2770_set_ivsense_transmit(struct tas2770_priv *tas2770,
 	return 0;
 }
 
+static int tas2770_set_pdm_transmit(struct tas2770_priv *tas2770, int slot)
+{
+	struct snd_soc_component *component = tas2770->component;
+	int ret;
+
+	ret = snd_soc_component_update_bits(component, TAS2770_TDM_CFG_REG7,
+					    TAS2770_TDM_CFG_REG7_PDM_MASK |
+					    TAS2770_TDM_CFG_REG7_50_MASK,
+					    TAS2770_TDM_CFG_REG7_PDM_ENABLE |
+					    slot);
+	return ret;
+}
+
 static int tas2770_set_bitwidth(struct tas2770_priv *tas2770, int bitwidth)
 {
 	int ret;
@@ -602,6 +615,13 @@ static int tas2770_codec_probe(struct snd_soc_component *component)
 			return ret;
 	}
 
+	if (tas2770->pdm_slot != -1) {
+		ret = tas2770_set_pdm_transmit(tas2770, tas2770->pdm_slot);
+
+		if (ret < 0)
+			return ret;
+	}
+
 	ret = snd_soc_component_update_bits(component, TAS2770_TDM_CFG_REG4,
 					    TAS2770_TDM_CFG_REG4_TX_FILL,
 					    tas2770->sdout_zfill ? 0 :
@@ -769,6 +789,12 @@ static int tas2770_parse_dt(struct device *dev, struct tas2770_priv *tas2770)
 		tas2770->v_sense_slot = -1;
 	}
 
+	rc = fwnode_property_read_u32(dev->fwnode, "ti,pdm-slot-no",
+				      &tas2770->pdm_slot);
+	if (rc) {
+		tas2770->pdm_slot = -1;
+	}
+
 	tas2770->sdout_pd = fwnode_property_read_bool(dev->fwnode, "ti,sdout-pull-down");
 	tas2770->sdout_zfill = fwnode_property_read_bool(dev->fwnode, "ti,sdout-zero-fill");
 

sound/soc/codecs/tas2770.h

@@ -85,6 +85,11 @@
 #define TAS2770_TDM_CFG_REG6_ISNS_MASK  BIT(6)
 #define TAS2770_TDM_CFG_REG6_ISNS_ENABLE  BIT(6)
 #define TAS2770_TDM_CFG_REG6_50_MASK  GENMASK(5, 0)
+    /* TDM Configuration Reg10 */
+#define TAS2770_TDM_CFG_REG7  TAS2770_REG(0X0, 0x11)
+#define TAS2770_TDM_CFG_REG7_PDM_MASK  BIT(6)
+#define TAS2770_TDM_CFG_REG7_PDM_ENABLE  BIT(6)
+#define TAS2770_TDM_CFG_REG7_50_MASK	GENMASK(5, 0)
     /* Brown Out Prevention Reg0 */
 #define TAS2770_BO_PRV_REG0  TAS2770_REG(0X0, 0x1B)
     /* Interrupt MASK Reg0 */
@@ -150,6 +155,7 @@ struct tas2770_priv {
 	struct device *dev;
 	int v_sense_slot;
 	int i_sense_slot;
+	int pdm_slot;
 	bool sdout_pd;
 	bool sdout_zfill;
 	bool dac_powered;