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