[PATCH net] net: stmmac: xgmac: do not treat TBU as a TX event

[PATCH net] net: stmmac: xgmac: do not treat TBU as a TX event

[Pengfei Zhang]

Commit 8d07a7930434 ("net: stmmac: xgmac: Do not enable TBU interrupt")
removed TBUE from the default DMA interrupt mask, but did not remove the
TBU check from dwxgmac2_dma_interrupt().

TBU is set whenever a TX channel runs out of descriptors. Since TBUE is
not enabled and only enabled bits are cleared, the TBU status bit is
never cleared. As a result, TBU stays set on every idle TX channel,
regardless of whether there is any TX traffic.

With a shared interrupt, stmmac_dma_interrupt() checks all channels,
so every interrupt from any source schedules TX NAPI on every idle TX
channel.

Only use TI to trigger TX processing.

Fixes: 8d07a7930434 ("net: stmmac: xgmac: Do not enable TBU interrupt")
Signed-off-by: Pengfei Zhang <zhangfeionline@gmail.com>
---
Tested on a board with 8 RX/TX DMA channels behind one shared
interrupt. A peer sends 10k UDP packets/s to the board and the board
sends nothing back, so there should be almost no TX work. Every idle
channel reads DMA_CH_STATUS 0x4 (TBU only).

Per second, averaged over 10 s:

                       before    after
  hw interrupts          9071     8580
  tx_normal_irq_n       72572        1
  tx_clean             ~73000        9

- hw interrupts: the shared interrupt line, from /proc/interrupts.
  Unchanged, the RX watchdog sets the rate.
- tx_normal_irq_n (ethtool -S): how often dwxgmac2_dma_interrupt()
  returned handle_tx. Before the fix every interrupt reports it for
  all 8 channels.
- tx_clean (ethtool -S): stmmac_tx_clean() runs, i.e. TX NAPI polls.
  Each spurious handle_tx schedules one. What is left after the fix
  is real TX and the TX coalesce timer. The "before" value is from a
  separate run with the same setup.

No packet loss before or after.

 drivers/net/ethernet/stmicro/stmmac/dwxgmac2_dma.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/dwxgmac2_dma.c b/drivers/net/ethernet/stmicro/stmmac/dwxgmac2_dma.c
index ff83858ebc..0a6e82a155 100644
--- a/drivers/net/ethernet/stmicro/stmmac/dwxgmac2_dma.c
+++ b/drivers/net/ethernet/stmicro/stmmac/dwxgmac2_dma.c
@@ -346,7 +346,7 @@ static int dwxgmac2_dma_interrupt(struct stmmac_priv *priv,
 		u64_stats_update_end(&stats->syncp);
 		ret |= handle_rx;
 	}
-	if (likely(intr_status & (XGMAC_TI | XGMAC_TBU))) {
+	if (likely(intr_status & XGMAC_TI)) {
 		u64_stats_update_begin(&stats->syncp);
 		u64_stats_inc(&stats->tx_normal_irq_n[chan]);
 		u64_stats_update_end(&stats->syncp);
-- 
2.54.0

Re: [PATCH net] net: stmmac: xgmac: do not treat TBU as a TX event

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - What hardware the change was tested on. For driver fixes please
   mention the device (and if relevant firmware version) used for
   testing, or say that the change was not tested on real hardware.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
other reasons, please include the information in the commit messages
then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.

Re: [PATCH net] net: stmmac: xgmac: do not treat TBU as a TX event

[Pengfei Zhang]

It has been tested on the hardware device (not publicly available),
a DWC XGMAC with 8 RX/TX DMA channels behind one shared interrupt.
The numbers below the --- in the patch are from that device.

Re: [PATCH net] net: stmmac: xgmac: do not treat TBU as a TX event

[Pengfei Zhang]

+Cc Jose Abreu, the author of the commit in Fixes:. I used an outdated
address for him in the original posting, sorry.

Re: [PATCH net] net: stmmac: xgmac: do not treat TBU as a TX event

[Maxime Chevallier]

Hi,

On 10/13/26 04:47, Pengfei Zhang wrote:
> Commit 8d07a7930434 ("net: stmmac: xgmac: Do not enable TBU interrupt")
> removed TBUE from the default DMA interrupt mask, but did not remove the
> TBU check from dwxgmac2_dma_interrupt().
> 
> TBU is set whenever a TX channel runs out of descriptors. Since TBUE is
> not enabled and only enabled bits are cleared, the TBU status bit is
> never cleared. As a result, TBU stays set on every idle TX channel,
> regardless of whether there is any TX traffic.
> 
> With a shared interrupt, stmmac_dma_interrupt() checks all channels,
> so every interrupt from any source schedules TX NAPI on every idle TX
> channel.
> 
> Only use TI to trigger TX processing.
> 
> Fixes: 8d07a7930434 ("net: stmmac: xgmac: Do not enable TBU interrupt")
> Signed-off-by: Pengfei Zhang <zhangfeionline@gmail.com>

Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>

> ---
> Tested on a board with 8 RX/TX DMA channels behind one shared
> interrupt. A peer sends 10k UDP packets/s to the board and the board
> sends nothing back, so there should be almost no TX work. Every idle
> channel reads DMA_CH_STATUS 0x4 (TBU only).
> 
> Per second, averaged over 10 s:
> 
>                        before    after
>   hw interrupts          9071     8580
>   tx_normal_irq_n       72572        1
>   tx_clean             ~73000        9
> 
> - hw interrupts: the shared interrupt line, from /proc/interrupts.
>   Unchanged, the RX watchdog sets the rate.
> - tx_normal_irq_n (ethtool -S): how often dwxgmac2_dma_interrupt()
>   returned handle_tx. Before the fix every interrupt reports it for
>   all 8 channels.
> - tx_clean (ethtool -S): stmmac_tx_clean() runs, i.e. TX NAPI polls.
>   Each spurious handle_tx schedules one. What is left after the fix
>   is real TX and the TX coalesce timer. The "before" value is from a
>   separate run with the same setup.
> 
> No packet loss before or after.

If you need to respin, that information should be in the commit log
directly.

Thanks,

Maxime