netdev - [PATCH v2 4/4] net: ravb: Ensure memory write completes before ringing TX doorbell

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Message-ID: <20251017151830.171062-5-prabhakar.mahadev-lad.rj@bp.renesas.com>
Date: Fri, 17 Oct 2025 16:18:30 +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>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Mitsuhiro Kimura <mitsuhiro.kimura.kc@...esas.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 v2 4/4] net: ravb: Ensure memory write completes before ringing TX doorbell

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

Add a final dma_wmb() barrier before triggering the transmit request
(TCCR_TSRQ) to ensure all descriptor and buffer writes are visible to
the DMA engine.

According to the hardware manual, a read-back operation is required
before writing to the doorbell register to guarantee completion of
previous writes. Instead of performing a dummy read, a dma_wmb() is
used to both enforce the same ordering semantics on the CPU side and
also to ensure completion of writes.

Fixes: c156633f1353 ("Renesas Ethernet AVB driver proper")
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>
---
v1->v2:
- New patch added to separate out the memory barrier change
  before ringing the doorbell.
---
 drivers/net/ethernet/renesas/ravb_main.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
index 0e40001f64b4..c3fc15f9ec85 100644
--- a/drivers/net/ethernet/renesas/ravb_main.c
+++ b/drivers/net/ethernet/renesas/ravb_main.c
@@ -2232,6 +2232,14 @@ static netdev_tx_t ravb_start_xmit(struct sk_buff *skb, struct net_device *ndev)
 		dma_wmb();
 		desc->die_dt = DT_FSINGLE;
 	}
+
+	/* Before ringing the doorbell we need to make sure that the latest
+	 * writes have been committed to memory, otherwise it could delay
+	 * things until the doorbell is rang again.
+	 * This is in replacement of the read operation mentioned in the HW
+	 * manuals.
+	 */
+	dma_wmb();
 	ravb_modify(ndev, TCCR, TCCR_TSRQ0 << q, TCCR_TSRQ0 << q);
 
 	priv->cur_tx[q] += num_tx_desc;
-- 
2.43.0