linux-kernel - [PATCH 3/3] net: ravb: Enforce descriptor type ordering to prevent early DMA start

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Message-ID: <20251015150026.117587-4-prabhakar.mahadev-lad.rj@bp.renesas.com>
Date: Wed, 15 Oct 2025 16:00:26 +0100
From: Prabhakar <prabhakar.csengg@...il.com>
To: Niklas Söderlund <niklas.soderlund@...natech.se>,
	Paul Barker <paul@...rker.dev>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
Cc: netdev@...r.kernel.org,
	linux-renesas-soc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Prabhakar <prabhakar.csengg@...il.com>,
	Biju Das <biju.das.jz@...renesas.com>,
	Fabrizio Castro <fabrizio.castro.jz@...esas.com>,
	Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>,
	stable@...r.kernel.org
Subject: [PATCH 3/3] net: ravb: Enforce descriptor type ordering to prevent early DMA start

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

Ensure TX descriptor type fields are written in a safe order so the DMA
engine does not begin processing a chain before all descriptors are
fully initialised.

For multi-descriptor transmissions the driver writes DT_FEND into the
last descriptor and DT_FSTART into the first. The DMA engine starts
processing when it sees DT_FSTART. If the compiler or CPU reorders the
writes and publishes DT_FSTART before DT_FEND, the DMA can start early
and process an incomplete chain, leading to corrupted transmissions or
DMA errors.

Fix this by writing DT_FEND before the dma_wmb() barrier, executing
dma_wmb() immediately before DT_FSTART (or DT_FSINGLE in the single
descriptor case), and then adding a wmb() after the type updates to
ensure CPU-side ordering before ringing the hardware doorbell.

On an RZ/G2L platform running an RT kernel, this reordering hazard was
observed as TX stalls and timeouts:

  [  372.968431] NETDEV WATCHDOG: end0 (ravb): transmit queue 0 timed out
  [  372.968494] WARNING: CPU: 0 PID: 10 at net/sched/sch_generic.c:467 dev_watchdog+0x4a4/0x4ac
  [  373.969291] ravb 11c20000.ethernet end0: transmit timed out, status 00000000, resetting...

This change enforces the required ordering and prevents the DMA engine
from observing DT_FSTART before the rest of the descriptor chain is
valid.

Fixes: 2f45d1902acf ("ravb: minimize TX data copying")
Cc: stable@...r.kernel.org
Co-developed-by: Fabrizio Castro <fabrizio.castro.jz@...esas.com>
Signed-off-by: Fabrizio Castro <fabrizio.castro.jz@...esas.com>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
---
 drivers/net/ethernet/renesas/ravb_main.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
index a200e205825a..2a995fa9bfff 100644
--- a/drivers/net/ethernet/renesas/ravb_main.c
+++ b/drivers/net/ethernet/renesas/ravb_main.c
@@ -2211,15 +2211,19 @@ 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();
+
+	/* For multi-descriptors set DT_FEND before calling dma_wmb() */
 	if (num_tx_desc > 1) {
 		desc->die_dt = DT_FEND;
 		desc--;
-		desc->die_dt = DT_FSTART;
-	} else {
-		desc->die_dt = DT_FSINGLE;
 	}
+
+	/* Descriptor type must be set after all the above writes */
+	dma_wmb();
+	desc->die_dt = (num_tx_desc > 1) ? DT_FSTART : DT_FSINGLE;
+
+	/* Ensure data is written to RAM before initiating DMA transfer */
+	wmb();
 	ravb_modify(ndev, TCCR, TCCR_TSRQ0 << q, TCCR_TSRQ0 << q);
 
 	priv->cur_tx[q] += num_tx_desc;
-- 
2.43.0