net: stmmac: fix taprio schedule configuration

When configuring a 802.1Qbv schedule through the tc taprio qdisc on an NXP
i.MX8MPlus device, the effective cycle time differed from the requested one
by N*96ns, with N number of entries in the Qbv Gate Control List. This is
because the driver was adding a 96ns margin to each interval of the GCL,
apparently to account for the IPG. The problem was observed on NXP
i.MX8MPlus devices but likely affected all devices relying on the same
configuration callback (dwmac 4.00, 4.10, 5.10 variants).

Fix the issue by removing the margins, and simply setup the MAC with the
provided cycle time value. This is the behavior expected by the user-space
API, as altering the Qbv schedule timings would break standards conformance.
This is also the behavior of several other Ethernet MAC implementations
supporting taprio, including the dwxgmac variant of stmmac.

Fixes: 504723a ("net: stmmac: Add basic EST support for GMAC5+")
Signed-off-by: Yannick Vignon <yannick.vignon@nxp.com>
Link: https://lore.kernel.org/r/20210113131557.24651-1-yannick.vignon@oss.nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/stmicro/stmmac/dwmac5.c

@@ -568,68 +568,24 @@ static int dwmac5_est_write(void __iomem *ioaddr, u32 reg, u32 val, bool gcl)
 int dwmac5_est_configure(void __iomem *ioaddr, struct stmmac_est *cfg,
 			 unsigned int ptp_rate)
 {
-	u32 speed, total_offset, offset, ctrl, ctr_low;
-	u32 extcfg = readl(ioaddr + GMAC_EXT_CONFIG);
-	u32 mac_cfg = readl(ioaddr + GMAC_CONFIG);
 	int i, ret = 0x0;
-	u64 total_ctr;
-
-	if (extcfg & GMAC_CONFIG_EIPG_EN) {
-		offset = (extcfg & GMAC_CONFIG_EIPG) >> GMAC_CONFIG_EIPG_SHIFT;
-		offset = 104 + (offset * 8);
-	} else {
-		offset = (mac_cfg & GMAC_CONFIG_IPG) >> GMAC_CONFIG_IPG_SHIFT;
-		offset = 96 - (offset * 8);
-	}
-
-	speed = mac_cfg & (GMAC_CONFIG_PS | GMAC_CONFIG_FES);
-	speed = speed >> GMAC_CONFIG_FES_SHIFT;
-
-	switch (speed) {
-	case 0x0:
-		offset = offset * 1000; /* 1G */
-		break;
-	case 0x1:
-		offset = offset * 400; /* 2.5G */
-		break;
-	case 0x2:
-		offset = offset * 100000; /* 10M */
-		break;
-	case 0x3:
-		offset = offset * 10000; /* 100M */
-		break;
-	default:
-		return -EINVAL;
-	}
-
-	offset = offset / 1000;
+	u32 ctrl;
 
 	ret |= dwmac5_est_write(ioaddr, BTR_LOW, cfg->btr[0], false);
 	ret |= dwmac5_est_write(ioaddr, BTR_HIGH, cfg->btr[1], false);
 	ret |= dwmac5_est_write(ioaddr, TER, cfg->ter, false);
 	ret |= dwmac5_est_write(ioaddr, LLR, cfg->gcl_size, false);
+	ret |= dwmac5_est_write(ioaddr, CTR_LOW, cfg->ctr[0], false);
+	ret |= dwmac5_est_write(ioaddr, CTR_HIGH, cfg->ctr[1], false);
 	if (ret)
 		return ret;
 
-	total_offset = 0;
 	for (i = 0; i < cfg->gcl_size; i++) {
-		ret = dwmac5_est_write(ioaddr, i, cfg->gcl[i] + offset, true);
+		ret = dwmac5_est_write(ioaddr, i, cfg->gcl[i], true);
 		if (ret)
 			return ret;
-
-		total_offset += offset;
 	}
 
-	total_ctr = cfg->ctr[0] + cfg->ctr[1] * 1000000000ULL;
-	total_ctr += total_offset;
-
-	ctr_low = do_div(total_ctr, 1000000000);
-
-	ret |= dwmac5_est_write(ioaddr, CTR_LOW, ctr_low, false);
-	ret |= dwmac5_est_write(ioaddr, CTR_HIGH, total_ctr, false);
-	if (ret)
-		return ret;
-
 	ctrl = readl(ioaddr + MTL_EST_CONTROL);
 	ctrl &= ~PTOV;
 	ctrl |= ((1000000000 / ptp_rate) * 6) << PTOV_SHIFT;