[PATCH net 3/3] net: stmmac: dwmac1000: stop hardware from dropping checksum-error packets

[Grische]

When the checksum offload engine (CoE) is enabled, the dwmac1000 MAC
silently drops any frame for which the engine detects a checksum error,
before the driver sees it.

This breaks VXLAN over IPv6 with zero UDP checksums, which RFC 6935 and
RFC 6936 allow for tunnels.

Set DMA_CONTROL_DT so that such frames are delivered instead of dropped,
as commit fe4042797651 ("net: stmmac: dwmac4: stop hardware from
dropping checksum-error packets") did for dwmac4. CoE remains enabled,
so the driver still sees the error status and marks the skb
CHECKSUM_NONE for the stack to verify.

Tested on an Aruba AP-325 (ipq806x-gmac-dwmac) with Rx checksum offload
on.

Link: https://lore.kernel.org/all/20250625132117.1b3264e8@xxxxxxxxxx/
Link: https://github.com/freifunk-gluon/gluon/issues/3783
Fixes: 47dd7a540b8a ("net: add support for STMicroelectronics Ethernet controllers.")
Cc: <stable@xxxxxxxxxxxxxxx> # 6.6.x: ee0aace5f844: net: stmmac: Correctly handle Rx checksum offload errors
Cc: <stable@xxxxxxxxxxxxxxx> # 6.6.x
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Grische <opensource@xxxxxxxxxxx>
---
 drivers/net/ethernet/stmicro/stmmac/dwmac1000.h     | 1 +
 drivers/net/ethernet/stmicro/stmmac/dwmac1000_dma.c | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac1000.h b/drivers/net/ethernet/stmicro/stmmac/dwmac1000.h
index 1de1f929d61c..004e6e30a68b 100644
--- a/drivers/net/ethernet/stmicro/stmmac/dwmac1000.h
+++ b/drivers/net/ethernet/stmicro/stmmac/dwmac1000.h
@@ -163,6 +163,7 @@ enum rx_tx_priority_ratio {
 
 /* DMA operation mode defines (start/stop tx/rx are placed in common header)*/
 /* Disable Drop TCP/IP csum error */
+#define DMA_CONTROL_DT		0x04000000
 #define DMA_CONTROL_RSF		0x02000000	/* Receive Store and Forward */
 #define DMA_CONTROL_DFF		0x01000000	/* Disaable flushing */
 /* Threshold for Activating the FC */
diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac1000_dma.c b/drivers/net/ethernet/stmicro/stmmac/dwmac1000_dma.c
index 3ac7a7949529..00c8d4dfb553 100644
--- a/drivers/net/ethernet/stmicro/stmmac/dwmac1000_dma.c
+++ b/drivers/net/ethernet/stmicro/stmmac/dwmac1000_dma.c
@@ -129,6 +129,8 @@ static void dwmac1000_dma_operation_mode_rx(struct stmmac_priv *priv,
 {
 	u32 csr6 = readl(ioaddr + DMA_CHAN_CONTROL(channel));
 
+	csr6 |= DMA_CONTROL_DT;
+
 	if (mode == SF_DMA_MODE) {
 		pr_debug("GMAC: enable RX store and forward mode\n");
 		csr6 |= DMA_CONTROL_RSF | DMA_CONTROL_DFF;
-- 
2.54.0