mb/siemens/mc_apl5: Correct the Tx signal from SATA port 0

Because of an incorrect transmit voltage swing, the signal must be
adjusted. The factor of slices for full swing level can be corrected via
the High Speed I/O Transmit Control Register 3. The appropriate value of
0.7 V was determined by using an oscilloscope.

Change-Id: I965960004ca44f1b37b16ce6484000fa7fd8ad90
Signed-off-by: Mario Scheithauer <mario.scheithauer@siemens.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76175
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jan Samek <jan.samek@siemens.com>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

src/mainboard/siemens/mc_apl1/variants/mc_apl5/mainboard.c

@@ -13,6 +13,9 @@
 #include <baseboard/variants.h>
 #include <types.h>
 
+#define TX_DWORD3_P0	0xc8c
+#define TX_SWING_MASK	0x00ff0000
+
 void variant_mainboard_final(void)
 {
 	struct device *dev = NULL;
@@ -32,6 +35,14 @@ void variant_mainboard_final(void)
 	 */
 	pcr_or32(PID_LPC, PCR_LPC_PRC, (PCR_LPC_CCE_EN | PCR_LPC_PCE_EN));
 
+	/*
+	 * Correct the SATA transmit signal via the High Speed I/O Transmit
+	 * Control Register 3 on SATA port 0.
+	 * Bit [23:16] sets the output voltage swing for TX line.
+	 * The value 0x5a sets the swing level to 0.7 V.
+	 */
+	pcr_rmw32(PID_MODPHY, TX_DWORD3_P0, ~TX_SWING_MASK, 0x5a << 16);
+
 	/* Set Master Enable for on-board PCI device if allowed. */
 	dev = dev_find_device(PCI_VID_SIEMENS, 0x403e, 0);
 	if (dev) {