[PATCH net] net: stmmac: dwmac-motorcomm: mask the MISC interrupt

[Magnus Leatti <magnusleatti@kernelriot.com>]

motorcomm_init() writes 0 to MGMT_INT_CTRL0, which unmasks the MISC
interrupt. MISC arrives on MSI-X vector 5, which stmmac requests as its
MAC interrupt, and nothing in stmmac clears it. The vendor driver masks
it.

Since commit 1b9707e6f1a9 ("net: stmmac: enable RPS and RBU interrupts"),
heavy RX load sets MISC and it never clears again, even though every
status register I could find reads zero. Vector 5 then fires about 120k
times per second and the machine stops responding until the TX watchdog
resets the adapter, or does not recover at all. A 64-byte UDP flood
from another host is enough:

  iperf3 -c <host> -u -b 0 -l 64 -R -P 4 -t 10

Mask MISC, as the vendor driver does. I could not find anything else
that depends on it here: EEE is not supported, and Wake-on-LAN uses PME.

Tested on one YT6801 board with net at cb284914e749. Without this
patch the flood above hangs the board within a second. With it, no MAC
interrupt fires under that flood or heavier ones, including after link,
ring, MTU and speed changes and a module reload, and throughput is
unchanged. Suspend could not be tested. The register values were read
with a debug build of stmmac.

Fixes: 02ff155ea281 ("net: stmmac: Add glue driver for Motorcomm YT6801 ethernet controller")
Signed-off-by: Magnus Leatti <magnusleatti@kernelriot.com>
---
Russell, d3265c19b35d ("net: stmmac: xgmac2: disable RBUE in default RX
interrupt mask") describes a similar storm on xgmac2. Should dwmac4 do
the same? I have only tested the glue change.

 drivers/net/ethernet/stmicro/stmmac/dwmac-motorcomm.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac-motorcomm.c b/drivers/net/ethernet/stmicro/stmmac/dwmac-motorcomm.c
index 124fcd55a..610d533b9 100644
--- a/drivers/net/ethernet/stmicro/stmmac/dwmac-motorcomm.c
+++ b/drivers/net/ethernet/stmicro/stmmac/dwmac-motorcomm.c
@@ -30,6 +30,7 @@
 #define OOB_WOL_CTRL				0x1010
 #define  OOB_WOL_CTRL_DIS			BIT(0)
 #define MGMT_INT_CTRL0				0x1100
+#define  MGMT_INT_CTRL0_MASK_MISC		BIT(21)
 #define INT_MODERATION				0x1108
 #define  INT_MODERATION_RX			GENMASK(11, 0)
 #define  INT_MODERATION_TX			GENMASK(27, 16)
@@ -172,7 +173,11 @@ static void motorcomm_reset(struct dwmac_motorcomm_priv *priv)
 
 static void motorcomm_init(struct dwmac_motorcomm_priv *priv)
 {
-	writel(0x0, priv->base + MGMT_INT_CTRL0);
+	/*
+	 * Nothing services the MISC source, and once raised it never
+	 * deasserts, so keep it masked.
+	 */
+	writel(MGMT_INT_CTRL0_MASK_MISC, priv->base + MGMT_INT_CTRL0);
 
 	writel(FIELD_PREP(INT_MODERATION_RX, 200) |
 	       FIELD_PREP(INT_MODERATION_TX, 200),

base-commit: cb284914e749388eef402938bdb3115fe1f6d337
-- 
2.54.0