Re: [PATCH v2 3/4] net: ravb: Enforce descriptor type ordering

["Niklas Söderlund" <niklas.soderlund@ragnatech.se>]

Hi Lad,

Thanks for reworking this and making it very clear what's going on.

On 2025-10-17 16:18:29 +0100, Prabhakar wrote:
> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
> 
> 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>

> ---
> v1->v2:
> - Reflowed the code and updated the comment to clarify the ordering
>   requirements.
> - Updated commit message.
> - Split up adding memory barrier change before ringing doorbell
>   into a separate patch.
> ---
>  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 a200e205825a..0e40001f64b4 100644
> --- a/drivers/net/ethernet/renesas/ravb_main.c
> +++ b/drivers/net/ethernet/renesas/ravb_main.c
> @@ -2211,13 +2211,25 @@ 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);
> -- 
> 2.43.0
> 

-- 
Kind Regards,
Niklas Söderlund