[PATCH net-next 5/7] net: qualcomm: rmnet: Work around coalescing hardware quirks

[Subash Abhinov Kasiviswanathan <subash.a.kasiviswanathan@oss.qualcomm.com>]

On certain hardware, the checksum valid indication from the bitmap
inside a MAPv5 coalescing frame cannot be trusted when the frame contains
exactly one packet and hardware completes the coalescing due to a
TCP FIN or PSH flag, a packet count limit, a byte count limit or a time
limit.  The hardware sets CSUM_VALID incorrectly in these cases, causing
the driver to mark packets CHECKSUM_UNNECESSARY when the checksum may in
fact be wrong.

Instead, set ip_summed to CHECKSUM_NONE so that the network stack verifies
the checksum rather than trusting the hardware indication.

The single NLO, single packet determination this fix depends on cannot
be based on the num_nlos field declared in the coalescing header, since
on certain simulation hardware configurations that field can be reported
incorrectly even though the per-NLO num_packets fields are accurate.
Recompute the true NLO count from the actual nl_pairs[] content before
applying the fixup so the determination is reliable regardless of
whether num_nlos itself can be trusted.

Co-developed-by: Sean Tranchetti <sean.tranchetti@oss.qualcomm.com>
Signed-off-by: Sean Tranchetti <sean.tranchetti@oss.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subash.a.kasiviswanathan@oss.qualcomm.com>
---
 .../ethernet/qualcomm/rmnet/rmnet_map_data.c  | 62 +++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
index bb88e19e28d8..1f9e592e24b6 100644
--- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
+++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
@@ -593,6 +593,61 @@ static void rmnet_map_partial_csum(struct sk_buff *skb,
 	skb->csum_start = skb->data + coal_meta->ip_len - skb->head;
 }
 
+/* On some hardware, num_nlos in the coalescing header can be reported
+ * incorrectly under certain conditions even though the per-NLO num_packets
+ * fields it is meant to summarize are correct. Recompute the true NLO count
+ * directly from the nl_pairs[] content rather than trusting the declared
+ * value, so that rmnet_map_v5_csum_fixup()'s single NLO, single packet check
+ * is reliable.
+ */
+static void rmnet_map_v5_fixup_num_nlos(struct rmnet_map_v5_coal_header *coal_hdr)
+{
+	u8 nlos = 0;
+	int i;
+
+	for (i = 0; i < RMNET_MAP_V5_MAX_NLOS; i++) {
+		if (coal_hdr->nl_pairs[i].num_packets)
+			nlos++;
+	}
+
+	coal_hdr->coal_info = u8_encode_bits(nlos, MAPV5_COALINFO_NUM_NLOS_FMASK) |
+			      (coal_hdr->coal_info & MAPV5_COALINFO_CSUM_VALID_FLAG);
+}
+
+/* The checksum valid indication for a single NLO, single packet coalescing
+ * frame cannot be trusted when the close reason is a TCP FIN/PSH, a packet
+ * count limit, a byte count limit or a time limit.
+ */
+static bool rmnet_map_v5_csum_fixup(struct rmnet_map_v5_coal_header *coal_hdr)
+{
+	u8 close_value = u8_get_bits(coal_hdr->close_info,
+				     MAPV5_CLOSEINFO_CLOSE_VALUE_FMASK);
+	u8 close_type = u8_get_bits(coal_hdr->close_info,
+				    MAPV5_CLOSEINFO_CLOSE_TYPE_FMASK);
+	u8 num_nlos = u8_get_bits(coal_hdr->coal_info,
+				  MAPV5_COALINFO_NUM_NLOS_FMASK);
+
+	/* Only applies to single NLO, single packet frames */
+	if (num_nlos != 1 || coal_hdr->nl_pairs[0].num_packets != 1)
+		return false;
+
+	/* TCP FIN or PSH triggered the close */
+	if (close_type == RMNET_MAP_COAL_CLOSE_COAL)
+		return true;
+
+	/* Hit a hardware limit */
+	if (close_type == RMNET_MAP_COAL_CLOSE_HW) {
+		switch (close_value) {
+		case RMNET_MAP_COAL_CLOSE_HW_PKT:
+		case RMNET_MAP_COAL_CLOSE_HW_BYTE:
+		case RMNET_MAP_COAL_CLOSE_HW_TIME:
+			return true;
+		}
+	}
+
+	return false;
+}
+
 /* Carve one logical segment from a coalesced SKB and append it to the list.
  * Adjusts TCP sequence numbers, IP IDs/lengths, and checksum state.
  */
@@ -913,6 +968,7 @@ static int rmnet_map_segment_coal_skb(struct sk_buff *coal_skb,
 	skb_pull(coal_skb, sizeof(struct rmnet_map_header));
 	skb_trim(coal_skb, len);
 	coal_hdr = (struct rmnet_map_v5_coal_header *)coal_skb->data;
+	rmnet_map_v5_fixup_num_nlos(coal_hdr);
 	num_nlos = u8_get_bits(coal_hdr->coal_info, MAPV5_COALINFO_NUM_NLOS_FMASK);
 	skb_pull(coal_skb, sizeof(*coal_hdr));
 
@@ -927,6 +983,12 @@ static int rmnet_map_segment_coal_skb(struct sk_buff *coal_skb,
 	if (!rmnet_map_coal_validate_bounds(coal_skb, coal_hdr, num_nlos, hlen))
 		return -EINVAL;
 
+	if (rmnet_map_v5_csum_fixup(coal_hdr) && !coal_meta.zero_csum) {
+		coal_skb->ip_summed = CHECKSUM_NONE;
+		__skb_queue_tail(list, coal_skb);
+		return 0;
+	}
+
 	if (rmnet_map_coal_gro_fast_path(coal_skb, coal_hdr, &coal_meta, list,
 					 num_nlos, gro))
 		return 0;
-- 
2.34.1