[PATCH 5.10.y] net: ravb: Enforce descriptor type ordering

[Sasha Levin <sashal@kernel.org>]

From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>

[ Upstream commit 5370c31e84b0e0999c7b5ff949f4e104def35584 ]

Ensure the TX descriptor type fields are published in a safe order so the
DMA engine never begins processing a descriptor chain before all descriptor
fields are fully initialised.

For multi-descriptor transmits the driver writes DT_FEND into the last
descriptor and DT_FSTART into the first. The DMA engine begins processing
when it observes DT_FSTART. Move the dma_wmb() barrier so it executes
immediately after DT_FEND and immediately before writing DT_FSTART
(and before DT_FSINGLE in the single-descriptor case). This guarantees
that all prior CPU writes to the descriptor memory are visible to the
device before DT_FSTART is seen.

This avoids a situation where compiler/CPU reordering could publish
DT_FSTART ahead of DT_FEND or other descriptor fields, allowing the DMA to
start on a partially initialised chain and causing corrupted transmissions
or TX timeouts. Such a failure was observed on RZ/G2L with an RT kernel as
transmit queue timeouts and device resets.

Fixes: 2f45d1902acf ("ravb: minimize TX data copying")
Cc: stable@vger.kernel.org
Co-developed-by: Fabrizio Castro <fabrizio.castro.jz@renesas.com>
Signed-off-by: Fabrizio Castro <fabrizio.castro.jz@renesas.com>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://patch.msgid.link/20251017151830.171062-4-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ kept unconditional skb_tx_timestamp() ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/net/ethernet/renesas/ravb_main.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
index b08478aabc6e6..7e71c60f75d5a 100644
--- a/drivers/net/ethernet/renesas/ravb_main.c
+++ b/drivers/net/ethernet/renesas/ravb_main.c
@@ -1596,13 +1596,25 @@ static netdev_tx_t ravb_start_xmit(struct sk_buff *skb, struct net_device *ndev)
 	}
 
 	skb_tx_timestamp(skb);
-	/* Descriptor type must be set after all the above writes */
-	dma_wmb();
+
 	if (num_tx_desc > 1) {
 		desc->die_dt = DT_FEND;
 		desc--;
+		/* When using multi-descriptors, DT_FEND needs to get written
+		 * before DT_FSTART, but the compiler may reorder the memory
+		 * writes in an attempt to optimize the code.
+		 * Use a dma_wmb() barrier to make sure DT_FEND and DT_FSTART
+		 * are written exactly in the order shown in the code.
+		 * This is particularly important for cases where the DMA engine
+		 * is already running when we are running this code. If the DMA
+		 * sees DT_FSTART without the corresponding DT_FEND it will enter
+		 * an error condition.
+		 */
+		dma_wmb();
 		desc->die_dt = DT_FSTART;
 	} else {
+		/* Descriptor type must be set after all the above writes */
+		dma_wmb();
 		desc->die_dt = DT_FSINGLE;
 	}
 	ravb_modify(ndev, TCCR, TCCR_TSRQ0 << q, TCCR_TSRQ0 << q);
-- 
2.51.0