[PATCH net v2 2/3] net: stmmac: dwmac1000: don't discard frames with only a checksum error

[Christian Schmidbauer <opensource@grische.xyz>]

On dwmac1000 with enhanced descriptors, the hardware sets the Error
Summary (ES) bit of the Rx descriptor not only for L2 errors but also
for an L3/L4 checksum error. enh_desc_get_rx_status() returns
discard_frame whenever ES is set, and since commit 1b746ce8b397 ("net:
stmmac: don't overwrite discard_frame status") the checksum status is
only evaluated for good frames.

Return discard_frame only when ES comes with one of the L2 error bits.
A frame with only a checksum error is then reported as csum_none and
reaches the stack as CHECKSUM_NONE, so the stack verifies the checksum
itself. This matches dwmac4, whose Error Summary bit does not cover
checksum errors.

RDES0 bit 7 is not in the mask, because depending on the core and on
Rx checksum offload it reports a timestamp, an IP header checksum error
or a giant frame, and enh_desc_get_rx_status() cannot tell which. In
threshold Rx mode, a giant frame that fits one buffer therefore reaches
the stack with a verified FCS.

Also remove the pr_err that enh_desc_get_rx_status() prints when bit 7
is set. Once the MAC stops dropping checksum-error frames, any host on
the link can flood the kernel log by sending frames with a bad IPv4
header checksum.

This prepares for setting DMA_CONTROL_DT, which makes the MAC deliver
checksum-error frames instead of dropping them in its Rx FIFO.

Fixes: 1b746ce8b397 ("net: stmmac: don't overwrite discard_frame status")
Cc: <stable@vger.kernel.org> # 6.6.x: ee0aace5f844: net: stmmac: Correctly handle Rx checksum offload errors
Cc: <stable@vger.kernel.org> # 6.6.x
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Christian Schmidbauer <opensource@grische.xyz>
---
v2:
- Drop RDES0_LENGTH_ERROR from the mask of Error Summary bits. The
  length check further down already discards such a frame.
- Explain in the commit message why RDES0 bit 7 is not treated as an
  L2 error.
v1: https://lore.kernel.org/netdev/20261020190319.884375-1-opensource@grische.xyz/
 drivers/net/ethernet/stmicro/stmmac/enh_desc.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/enh_desc.c b/drivers/net/ethernet/stmicro/stmmac/enh_desc.c
index 7ece8d4157a8..d0622f2605aa 100644
--- a/drivers/net/ethernet/stmicro/stmmac/enh_desc.c
+++ b/drivers/net/ethernet/stmicro/stmmac/enh_desc.c
@@ -216,9 +216,6 @@ static int enh_desc_get_rx_status(struct stmmac_extra_stats *x,
 		if (unlikely(rdes0 & RDES0_OVERFLOW_ERROR))
 			x->rx_gmac_overflow++;
 
-		if (unlikely(rdes0 & RDES0_IPC_CSUM_ERROR))
-			pr_err("\tIPC Csum Error/Giant frame\n");
-
 		if (unlikely(rdes0 & RDES0_COLLISION))
 			x->rx_collision++;
 		if (unlikely(rdes0 & RDES0_RECEIVE_WATCHDOG))
@@ -230,7 +227,10 @@ static int enh_desc_get_rx_status(struct stmmac_extra_stats *x,
 		if (unlikely(rdes0 & RDES0_CRC_ERROR)) {
 			x->rx_crc_errors++;
 		}
-		ret = discard_frame;
+		if (rdes0 & (RDES0_DESCRIPTOR_ERROR | RDES0_OVERFLOW_ERROR |
+			     RDES0_COLLISION | RDES0_RECEIVE_WATCHDOG |
+			     RDES0_MII_ERROR | RDES0_CRC_ERROR))
+			ret = discard_frame;
 	}
 
 	/* After a payload csum error, the ES bit is set.
-- 
2.54.0