linux-kernel - [PATCH net] net: stmmac: dwmac-socfpga: Set RX watchdog interrupt as broken

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Message-ID: <20241122141256.764578-1-maxime.chevallier@bootlin.com>
Date: Fri, 22 Nov 2024 15:12:55 +0100
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Jose Abreu <joabreu@...opsys.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	davem@...emloft.net,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>
Cc: Maxime Chevallier <maxime.chevallier@...tlin.com>,
	Alexis Lothoré <alexis.lothore@...tlin.com>,
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
	netdev@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH net] net: stmmac: dwmac-socfpga: Set RX watchdog interrupt as broken

On DWMAC3 and later, there's a RX Watchdog interrupt that's used for
interrupt coalescing. It's known to be buggy on some platforms, and
dwmac-socfpga appears to be one of them. Changing the interrupt
coalescing from ethtool doesn't appear to have any effect here.

Without disabling RIWT (Received Interrupt Watchdog Timer, I
believe...), we observe latencies while receiving traffic that amount to
around ~0.4ms. This was discovered with NTP but can be easily reproduced
with a simple ping. Without this patch :

64 bytes from 192.168.5.2: icmp_seq=1 ttl=64 time=0.657 ms

With this patch :

64 bytes from 192.168.5.2: icmp_seq=1 ttl=64 time=0.254 ms

Fixes: 801d233b7302 ("net: stmmac: Add SOCFPGA glue driver")
Signed-off-by: Maxime Chevallier <maxime.chevallier@...tlin.com>
---
 drivers/net/ethernet/stmicro/stmmac/dwmac-socfpga.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac-socfpga.c b/drivers/net/ethernet/stmicro/stmmac/dwmac-socfpga.c
index 248b30d7b864..16020b72dec8 100644
--- a/drivers/net/ethernet/stmicro/stmmac/dwmac-socfpga.c
+++ b/drivers/net/ethernet/stmicro/stmmac/dwmac-socfpga.c
@@ -487,6 +487,8 @@ static int socfpga_dwmac_probe(struct platform_device *pdev)
 	plat_dat->select_pcs = socfpga_dwmac_select_pcs;
 	plat_dat->has_gmac = true;
 
+	plat_dat->riwt_off = 1;
+
 	ret = stmmac_dvr_probe(&pdev->dev, plat_dat, &stmmac_res);
 	if (ret)
 		return ret;
-- 
2.47.0