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

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Message-ID: <20251022121614.GE1694476@ragnatech.se>
Date: Wed, 22 Oct 2025 14:16:14 +0200
From: Niklas Söderlund <niklas.soderlund@...natech.se>
To: Prabhakar <prabhakar.csengg@...il.com>
Cc: 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>,
	netdev@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
	linux-kernel@...r.kernel.org, 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: Re: [PATCH v2 4/4] net: ravb: Ensure memory write completes before
 ringing TX doorbell

Hello Lad,

Thanks for your work.

On 2025-10-17 16:18:30 +0100, Prabhakar wrote:
> 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>

Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@...natech.se>

> ---
> 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

nit: I would spell out hardware here, if you do a v3.

> +	 * manuals.
> +	 */
> +	dma_wmb();
>  	ravb_modify(ndev, TCCR, TCCR_TSRQ0 << q, TCCR_TSRQ0 << q);
>  
>  	priv->cur_tx[q] += num_tx_desc;
> -- 
> 2.43.0
> 

-- 
Kind Regards,
Niklas Söderlund