realtek: dsa: rtl930x: Fix flow control with ingress shaping

Tests with ingress shaping and enabled flow control showed really high
packet loss. It seems like the MAC pause frames are not created correctly
when both burst high off is set to the same value as burst high on.

By default, RTL930x has set the burst high values to:

* on: 64K
* off: 32K

Using the same 1:2 ratio seems to solve the high packet loss rate during
UDP tests.

Fixes: 2e74eb6d93a7 ("realtek: dsa: rtl93xx: Support per port throttling")
Signed-off-by: Sven Eckelmann <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/21011
Signed-off-by: Robert Marko <[email protected]>

target/linux/realtek/files-6.12/drivers/net/dsa/rtl83xx/rtl930x.c

@@ -208,9 +208,13 @@ static int rtldsa_930x_port_rate_police_add(struct dsa_switch *ds, int port,
 	if (ingress) {
 		burst = min_t(u32, act->police.burst, RTL930X_BANDWIDTH_CTRL_INGRESS_BURST_MAX);
 
-		/* set burst high on/off the same to avoid TCP oscillation */
+		/* the linux kernel only provides a single burst value. But the
+		 * realtek HW needs two. And to get flow control correctly
+		 * working, the realtek default ratio of 1:2 seems to work
+		 * reasonable well
+		 */
 		sw_w32(burst, RTL930X_BANDWIDTH_CTRL_INGRESS_BURST_HIGH_ON(port));
-		sw_w32(burst, RTL930X_BANDWIDTH_CTRL_INGRESS_BURST_HIGH_OFF(port));
+		sw_w32(burst / 2, RTL930X_BANDWIDTH_CTRL_INGRESS_BURST_HIGH_OFF(port));
 
 		/* Enable ingress bandwidth flow control to improve TCP throughput and avoid
 		 * the drops behavior of the RTL930x ingress rate limiter which seem to not