thunderbolt: Honor TMU requirements in the domain when setting TMU mode

Currently, when configuring TMU (Time Management Unit) mode of a given
router, we take into account only its own TMU requirements ignoring
other routers in the domain. This is problematic if the router we are
configuring has lower TMU requirements than what is already configured
in the domain.

In the scenario below, we have a host router with two USB4 ports: A and
B. Port A connected to device router #1 (which supports CL states) and
existing DisplayPort tunnel, thus, the TMU mode is HiFi uni-directional.

1. Initial topology

          [Host]
         A/
         /
 [Device #1]
   /
Monitor

2. Plug in device #2 (that supports CL states) to downstream port B of
   the host router

         [Host]
        A/    B\
        /       \
 [Device #1]    [Device #2]
   /
Monitor

The TMU mode on port B and port A will be configured to LowRes which is
not what we want and will cause monitor to start flickering.

To address this we first scan the domain and search for any router
configured to HiFi uni-directional mode, and if found, configure TMU
mode of the given router to HiFi uni-directional as well.

Cc: [email protected]
Signed-off-by: Gil Fine <[email protected]>
Signed-off-by: Mika Westerberg <[email protected]>

drivers/thunderbolt/tb.c

@@ -288,6 +288,24 @@ static void tb_increase_tmu_accuracy(struct tb_tunnel *tunnel)
 	device_for_each_child(&sw->dev, NULL, tb_increase_switch_tmu_accuracy);
 }
 
+static int tb_switch_tmu_hifi_uni_required(struct device *dev, void *not_used)
+{
+	struct tb_switch *sw = tb_to_switch(dev);
+
+	if (sw && tb_switch_tmu_is_enabled(sw) &&
+	    tb_switch_tmu_is_configured(sw, TB_SWITCH_TMU_MODE_HIFI_UNI))
+		return 1;
+
+	return device_for_each_child(dev, NULL,
+				     tb_switch_tmu_hifi_uni_required);
+}
+
+static bool tb_tmu_hifi_uni_required(struct tb *tb)
+{
+	return device_for_each_child(&tb->dev, NULL,
+				     tb_switch_tmu_hifi_uni_required) == 1;
+}
+
 static int tb_enable_tmu(struct tb_switch *sw)
 {
 	int ret;
@@ -302,12 +320,30 @@ static int tb_enable_tmu(struct tb_switch *sw)
 	ret = tb_switch_tmu_configure(sw,
 			TB_SWITCH_TMU_MODE_MEDRES_ENHANCED_UNI);
 	if (ret == -EOPNOTSUPP) {
-		if (tb_switch_clx_is_enabled(sw, TB_CL1))
-			ret = tb_switch_tmu_configure(sw,
-					TB_SWITCH_TMU_MODE_LOWRES);
-		else
-			ret = tb_switch_tmu_configure(sw,
-					TB_SWITCH_TMU_MODE_HIFI_BI);
+		if (tb_switch_clx_is_enabled(sw, TB_CL1)) {
+			/*
+			 * Figure out uni-directional HiFi TMU requirements
+			 * currently in the domain. If there are no
+			 * uni-directional HiFi requirements we can put the TMU
+			 * into LowRes mode.
+			 *
+			 * Deliberately skip bi-directional HiFi links
+			 * as these work independently of other links
+			 * (and they do not allow any CL states anyway).
+			 */
+			if (tb_tmu_hifi_uni_required(sw->tb))
+				ret = tb_switch_tmu_configure(sw,
+						TB_SWITCH_TMU_MODE_HIFI_UNI);
+			else
+				ret = tb_switch_tmu_configure(sw,
+						TB_SWITCH_TMU_MODE_LOWRES);
+		} else {
+			ret = tb_switch_tmu_configure(sw, TB_SWITCH_TMU_MODE_HIFI_BI);
+		}
+
+		/* If not supported, fallback to bi-directional HiFi */
+		if (ret == -EOPNOTSUPP)
+			ret = tb_switch_tmu_configure(sw, TB_SWITCH_TMU_MODE_HIFI_BI);
 	}
 	if (ret)
 		return ret;