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

[Pengfei Zhang <zhangfeionline@gmail.com>]

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