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

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

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