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: 801d233 ("net: stmmac: Add SOCFPGA glue driver")
Signed-off-by: Maxime Chevallier <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: minimaxwell

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;