SoundWire: Allow Prepare command for Simplified_CP_SM

As defined in the MIPI SoundWire specification v1-2 for
Simplified Channel Prepare State Machine (Simplified_CP_SM):

* Figure 141 for the Simplified_CP_SM in the specification
  shows the "Ready" state (NF=0, P=1) that can be reached via
  "Prepare0 OR Prepare1" transitions.
* Table 115 (Stimulus to the Channel Prepare State Machine)
  indicates that Prepare0 and Prepare1 are read-only/"write-ignored"
  bits for Simplified_CP_SM.

In TI device implementations, we've found that some devices with
Simplified_CP_SM still benefit from receiving the Prepare command.

This patch modifies the code to:
1. Send the Prepare command to all devices, including those with
   Simplified_CP_SM
2. Ignore errors returned by devices with Simplified_CP_SM that
   might not support this command

This approach maintains compatibility with all devices while ensuring
proper functionality of dataport operations for devices that can
make use of the Prepare command despite using Simplified_CP_SM.

Signed-off-by: Niranjan H Y <niranjan.hy@ti.com>

Author: niranjanhyti

drivers/soundwire/stream.c

@@ -512,14 +512,19 @@ static int sdw_prep_deprep_slave_ports(struct sdw_bus *bus,
 	sdw_do_port_prep(s_rt, prep_ch, prep ? SDW_OPS_PORT_PRE_PREP : SDW_OPS_PORT_PRE_DEPREP);
 
 	/* Prepare Slave port implementing CP_SM */
-	if (!simple_ch_prep_sm) {
-		addr = SDW_DPN_PREPARECTRL(p_rt->num);
-
-		if (prep)
-			ret = sdw_write_no_pm(s_rt->slave, addr, p_rt->ch_mask);
-		else
-			ret = sdw_write_no_pm(s_rt->slave, addr, 0x0);
+	/* For Simplified_CP_SM, MIPI SoundWire specification v1-2 indicates
+	 * Prepare bits are "write-ignored" - this means devices may ignore the command.
+	 * Some devices still benefit from receiving this command even when using
+	 * Simplified_CP_SM, so we send it to all devices and ignore errors from those
+	 * that don't support it.
+	 */
+	addr = SDW_DPN_PREPARECTRL(p_rt->num);
+	if (prep)
+		ret = sdw_write_no_pm(s_rt->slave, addr, p_rt->ch_mask);
+	else
+		ret = sdw_write_no_pm(s_rt->slave, addr, 0x0);
 
+	if (!simple_ch_prep_sm) {
 		if (ret < 0) {
 			dev_err(&s_rt->slave->dev,
 				"Slave prep_ctrl reg write failed\n");
@@ -538,6 +543,11 @@ static int sdw_prep_deprep_slave_ports(struct sdw_bus *bus,
 				"Chn prep failed for port %d: %d\n", prep_ch.num, ret);
 			return ret;
 		}
+	} else {
+		/* Some device return error for the prepare command,
+		 * ignore the error for Simplified CP_SM
+		 */
+		ret = 0;
 	}
 
 	/* Inform slaves about ports prepared */