[coreboot-gerrit] Change in coreboot[master]: drivers: sn65dsi86: Retry link training up to 5 times

[Julius Werner (Code Review)]

Hello Douglas Anderson, Philip Chen, mturney mturney,

I'd like you to do a code review. Please visit

https://review.coreboot.org/c/coreboot/+/45882

to review the following change.

Change subject: drivers: sn65dsi86: Retry link training up to 5 times
......................................................................

drivers: sn65dsi86: Retry link training up to 5 times

The kernel guys have found that automatic link training from this bridge
can occasionally fail and needs to be retried. They think 5 retries
should be enough but have added up to 10 to be sure. Let's do the same,
but since things are synchronous for us and every try takes 500ms, maybe
better restrict it to 5.

BUG=b:169535092

Signed-off-by: Julius Werner 
Change-Id: I713b6851bd51d3527ed4c6e6407dee6b42d09955
---
M src/drivers/ti/sn65dsi86bridge/sn65dsi86bridge.c
1 file changed, 9 insertions(+), 6 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/82/45882/1

diff --git a/src/drivers/ti/sn65dsi86bridge/sn65dsi86bridge.c b/src/drivers/ti/sn65dsi86bridge/sn65dsi86bridge.c
index e0058c4..91e64aa 100644
--- a/src/drivers/ti/sn65dsi86bridge/sn65dsi86bridge.c
+++ b/src/drivers/ti/sn65dsi86bridge/sn65dsi86bridge.c
@@ -415,15 +415,18 @@
    sn65dsi86_bridge_dpcd_request(bus, chip,
    			      DP_BRIDGE_CONFIGURATION_SET, 1, DPCD_WRITE, &buf);

-	/* semi auto link training mode */
-	i2c_writeb(bus, chip, SN_ML_TX_MODE_REG, 0xa);
+	int i;	/* Kernel driver suggests to retry this a few times if it fails. */
+	for (i = 0; i < 5; i++) {
+		i2c_writeb(bus, chip, SN_ML_TX_MODE_REG, SEMI_AUTO_LINK_TRAINING);

-	if (!wait_ms(500,
-	    !(i2c_readb(bus, chip, SN_ML_TX_MODE_REG, &buf)) &&
-	    (buf & NORMAL_MODE))) {
-		printk(BIOS_ERR, "ERROR: Link training failed");
+		wait_ms(500, !(i2c_readb(bus, chip, SN_ML_TX_MODE_REG, &buf)) &&
+			     (buf == NORMAL_MODE || buf == MAIN_LINK_OFF))
+
+		if (buf == NORMAL_MODE)
+			return;
    }

+	printk(BIOS_ERR, "ERROR: Link training failed 5 times\n");
 }

static enum cb_err sn65dsi86_bridge_get_plug_in_status(uint8_t bus, uint8_t chip)