net: stmmac: correct MAC propagation delay

The IEEE1588 Standard specifies that the timestamps of Packets must be
captured when the PTP message timestamp point (leading edge of first
octet after the start of frame delimiter) crosses the boundary between
the node and the network. As the MAC latches the timestamp at an
internal point, the captured timestamp must be corrected for the
additional path latency, as described in the publicly available
datasheet [1].

This patch only corrects for the MAC-Internal delay, which can be read
out from the MAC_Ingress_Timestamp_Latency register, since the Phy
framework currently does not support querying the Phy ingress and egress
latency. The Closs Domain Crossing Circuits errors as indicated in [1]
are already being accounted in the stmmac_get_tx_hwtstamp() function and
are not corrected here.

As the Latency varies for different link speeds and MII
modes of operation, the correction value needs to be updated on each
link state change.

As the delay also causes a phase shift in the timestamp counter compared
to the rest of the network, this correction will also reduce phase error
when generating PPS outputs from the timestamp counter.

[1] i.MX8MP Reference Manual, rev.1 Section 11.7.2.5.3 "Timestamp
correction"

Signed-off-by: Johannes Zink <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/stmicro/stmmac/hwif.h

···
 	void (*get_systime) (void __iomem *ioaddr, u64 *systime);
 	void (*get_ptptime)(void __iomem *ioaddr, u64 *ptp_time);
 	void (*timestamp_interrupt)(struct stmmac_priv *priv);
+	void (*correct_latency)(struct stmmac_priv *priv);
 };
 
 #define stmmac_config_hw_tstamping(__priv, __args...) \
···
 	stmmac_do_void_callback(__priv, ptp, get_ptptime, __args)
 #define stmmac_timestamp_interrupt(__priv, __args...) \
 	stmmac_do_void_callback(__priv, ptp, timestamp_interrupt, __args)
+#define stmmac_correct_latency(__priv, __args...) \
+	stmmac_do_void_callback(__priv, ptp, correct_latency, __args)
 
 struct stmmac_tx_queue;
 struct stmmac_rx_queue;

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

···
 		*ssinc = data;
 }
 
+static void correct_latency(struct stmmac_priv *priv)
+{
+	void __iomem *ioaddr = priv->ptpaddr;
+	u32 reg_tsic, reg_tsicsns;
+	u32 reg_tsec, reg_tsecsns;
+	u64 scaled_ns;
+	u32 val;
+
+	/* MAC-internal ingress latency */
+	scaled_ns = readl(ioaddr + PTP_TS_INGR_LAT);
+
+	/* See section 11.7.2.5.3.1 "Ingress Correction" on page 4001 of
+	 * i.MX8MP Applications Processor Reference Manual Rev. 1, 06/2021
+	 */
+	val = readl(ioaddr + PTP_TCR);
+	if (val & PTP_TCR_TSCTRLSSR)
+		/* nanoseconds field is in decimal format with granularity of 1ns/bit */
+		scaled_ns = ((u64)NSEC_PER_SEC << 16) - scaled_ns;
+	else
+		/* nanoseconds field is in binary format with granularity of ~0.466ns/bit */
+		scaled_ns = ((1ULL << 31) << 16) -
+			DIV_U64_ROUND_CLOSEST(scaled_ns * PSEC_PER_NSEC, 466U);
+
+	reg_tsic = scaled_ns >> 16;
+	reg_tsicsns = scaled_ns & 0xff00;
+
+	/* set bit 31 for 2's compliment */
+	reg_tsic |= BIT(31);
+
+	writel(reg_tsic, ioaddr + PTP_TS_INGR_CORR_NS);
+	writel(reg_tsicsns, ioaddr + PTP_TS_INGR_CORR_SNS);
+
+	/* MAC-internal egress latency */
+	scaled_ns = readl(ioaddr + PTP_TS_EGR_LAT);
+
+	reg_tsec = scaled_ns >> 16;
+	reg_tsecsns = scaled_ns & 0xff00;
+
+	writel(reg_tsec, ioaddr + PTP_TS_EGR_CORR_NS);
+	writel(reg_tsecsns, ioaddr + PTP_TS_EGR_CORR_SNS);
+}
+
 static int init_systime(void __iomem *ioaddr, u32 sec, u32 nsec)
 {
 	u32 value;
···
 	.get_systime = get_systime,
 	.get_ptptime = get_ptptime,
 	.timestamp_interrupt = timestamp_interrupt,
+	.correct_latency = correct_latency,
 };

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

···
 	priv->hwts_tx_en = 0;
 	priv->hwts_rx_en = 0;
 
+	stmmac_correct_latency(priv, priv);
+
 	return 0;
 }
 
···
 
 	if (priv->dma_cap.fpesel)
 		stmmac_fpe_link_state_handle(priv, true);
+
+	stmmac_correct_latency(priv, priv);
 }
 
 static const struct phylink_mac_ops stmmac_phylink_mac_ops = {

drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.h

···
 #define	PTP_ACR		0x40	/* Auxiliary Control Reg */
 #define	PTP_ATNR	0x48	/* Auxiliary Timestamp - Nanoseconds Reg */
 #define	PTP_ATSR	0x4c	/* Auxiliary Timestamp - Seconds Reg */
+#define	PTP_TS_INGR_CORR_NS	0x58	/* Ingress timestamp correction nanoseconds */
+#define	PTP_TS_EGR_CORR_NS	0x5C	/* Egress timestamp correction nanoseconds*/
+#define	PTP_TS_INGR_CORR_SNS	0x60	/* Ingress timestamp correction subnanoseconds */
+#define	PTP_TS_EGR_CORR_SNS	0x64	/* Egress timestamp correction subnanoseconds */
+#define	PTP_TS_INGR_LAT	0x68	/* MAC internal Ingress Latency */
+#define	PTP_TS_EGR_LAT	0x6c	/* MAC internal Egress Latency */
 
 #define	PTP_STNSUR_ADDSUB_SHIFT	31
 #define	PTP_DIGITAL_ROLLOVER_MODE	0x3B9ACA00	/* 10e9-1 ns */