net: stmmac: xgmac2: disable RBUE in default RX interrupt mask

Enabling the RX Buffer Unavailable (RBUE) interrupt is counterproductive
and can trigger a MAC interrupt storm under heavy RX pressure. When the
DMA runs out of RX descriptors it fires RBUE continuously until software
refills the ring.

However, RBUE is redundant: the normal RX completion interrupt (RIE)
already triggers NAPI, which processes completed descriptors and refills
the ring, causing the DMA to resume. The RBUE handler itself only sets
handle_rx - the same outcome as RIE.

On Agilex5 under heavy RX pressure, the MAC interrupt (which includes
RBUE) was observed firing 1,821,811,555 times against only 2,618,627
actual RX completions - a ~695x ratio - confirming the severity of the
storm.

RBUE does not provide OOM recovery. If page_pool is exhausted,
stmmac_rx_refill() cannot advance the DMA tail pointer, the DMA stays
suspended, and RBUE fires again on the next NAPI completion - a storm
with no forward progress. This patch trades that storm for a clean
stall with the same RX outcome. Proper OOM recovery is a pre-existing
gap outside the scope of this fix.

Note: as a consequence of disabling RBUE, the rx_buf_unav_irq ethtool
counter will always read 0 on XGMAC2 devices. This behaviour is already
inconsistent across DWMAC core versions.

Remove RBUE from XGMAC_DMA_INT_DEFAULT_EN and XGMAC_DMA_INT_DEFAULT_RX
to prevent the interrupt storm while keeping normal RX handling intact.

Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: Nazim Amirul <muhammad.nazim.amirul.nazle.asmade@altera.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260609121703.9736-1-muhammad.nazim.amirul.nazle.asmade@altera.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/ethernet/stmicro/stmmac/dwxgmac2.h b/drivers/net/ethernet/stmicro/stmmac/dwxgmac2.h
index 95fdf3133208751546776cd625df244111ef56c6..61b6d45a02f5d52ef3b5fe53441ec4ae6002d0b3 100644 (file)
--- a/drivers/net/ethernet/stmicro/stmmac/dwxgmac2.h
+++ b/drivers/net/ethernet/stmicro/stmmac/dwxgmac2.h
@@ -379,9 +379,9 @@
 #define XGMAC_RIE                      BIT(6)
 #define XGMAC_TBUE                     BIT(2)
 #define XGMAC_TIE                      BIT(0)
-#define XGMAC_DMA_INT_DEFAULT_EN       (XGMAC_NIE | XGMAC_AIE | XGMAC_RBUE | \
+#define XGMAC_DMA_INT_DEFAULT_EN       (XGMAC_NIE | XGMAC_AIE | \
                                        XGMAC_RIE | XGMAC_TIE)
-#define XGMAC_DMA_INT_DEFAULT_RX       (XGMAC_RBUE | XGMAC_RIE)
+#define XGMAC_DMA_INT_DEFAULT_RX       (XGMAC_RIE)
 #define XGMAC_DMA_INT_DEFAULT_TX       (XGMAC_TIE)
 #define XGMAC_DMA_CH_Rx_WATCHDOG(x)    (0x0000313c + (0x80 * (x)))
 #define XGMAC_RWT                      GENMASK(7, 0)