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

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

Hi Prabhakar,

Thanks for your work.

On 2025-10-15 16:00:26 +0100, Prabhakar wrote:
> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.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@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>
> ---
>  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;

IMHO it's ugly to evaluate num_tx_desc twice. I would rather just open 
code the full steps in each branch of the if above. It would make it 
easier to read and understand.

> +
> +	/* Ensure data is written to RAM before initiating DMA transfer */
> +	wmb();

All of this looks a bit odd, why not just do a single dma_wmb() or wmb() 
before ringing the doorbell? Maybe I'm missing something obvious?

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

-- 
Kind Regards,
Niklas Söderlund