drm/bridge: ti-sn65dsi86: Update reply on aux failures

We should be setting the drm_dp_aux_msg::reply field if a NACK or a
SHORT reply happens. Update the error bit handling logic in
ti_sn_aux_transfer() to handle these cases and notify upper layers that
such errors have happened. This helps the retry logic understand that a
timeout has happened, or to shorten the read length if the panel isn't
able to handle the longest read possible.

Note: I don't have any hardware that exhibits these code paths so this
is written based on reading the datasheet for this bridge and inspecting
the code and how this is called.

Changes in v2:
 - Move WRITE_STATUS_UPDATE check from case to assignment

Changes in v2:
 - Handle WRITE_STATUS_UPDATE properly
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Jonas Karlman <jonas@kwiboo.se>
Cc: Jernej Skrabec <jernej.skrabec@siol.net>
Cc: Sean Paul <seanpaul@chromium.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201102181144.3469197-5-swboyd@chromium.org

drivers/gpu/drm/bridge/ti-sn65dsi86.c

@@ -878,7 +878,7 @@ static ssize_t ti_sn_aux_transfer(struct drm_dp_aux *aux,
 				  struct drm_dp_aux_msg *msg)
 {
 	struct ti_sn_bridge *pdata = aux_to_ti_sn_bridge(aux);
-	u32 request = msg->request & ~DP_AUX_I2C_MOT;
+	u32 request = msg->request & ~(DP_AUX_I2C_MOT | DP_AUX_I2C_WRITE_STATUS_UPDATE);
 	u32 request_val = AUX_CMD_REQ(msg->request);
 	u8 *buf = msg->buffer;
 	unsigned int len = msg->size;
@@ -895,6 +895,8 @@ static ssize_t ti_sn_aux_transfer(struct drm_dp_aux *aux,
 	case DP_AUX_NATIVE_READ:
 	case DP_AUX_I2C_READ:
 		regmap_write(pdata->regmap, SN_AUX_CMD_REG, request_val);
+		/* Assume it's good */
+		msg->reply = 0;
 		break;
 	default:
 		return -EINVAL;
@@ -926,10 +928,33 @@ static ssize_t ti_sn_aux_transfer(struct drm_dp_aux *aux,
 	ret = regmap_read(pdata->regmap, SN_AUX_CMD_STATUS_REG, &val);
 	if (ret)
 		return ret;
-	else if ((val & AUX_IRQ_STATUS_NAT_I2C_FAIL)
-		 || (val & AUX_IRQ_STATUS_AUX_RPLY_TOUT)
-		 || (val & AUX_IRQ_STATUS_AUX_SHORT))
-		return -ENXIO;
+
+	if (val & AUX_IRQ_STATUS_AUX_RPLY_TOUT) {
+		/*
+		 * The hardware tried the message seven times per the DP spec
+		 * but it hit a timeout. We ignore defers here because they're
+		 * handled in hardware.
+		 */
+		return -ETIMEDOUT;
+	}
+
+	if (val & AUX_IRQ_STATUS_AUX_SHORT) {
+		ret = regmap_read(pdata->regmap, SN_AUX_LENGTH_REG, &len);
+		if (ret)
+			return ret;
+	} else if (val & AUX_IRQ_STATUS_NAT_I2C_FAIL) {
+		switch (request) {
+		case DP_AUX_I2C_WRITE:
+		case DP_AUX_I2C_READ:
+			msg->reply |= DP_AUX_I2C_REPLY_NACK;
+			break;
+		case DP_AUX_NATIVE_READ:
+		case DP_AUX_NATIVE_WRITE:
+			msg->reply |= DP_AUX_NATIVE_REPLY_NACK;
+			break;
+		}
+		return 0;
+	}
 
 	if (request == DP_AUX_NATIVE_WRITE || request == DP_AUX_I2C_WRITE ||
 	    len == 0)