[PATCH net-next v6 1/3] net: stmmac: dwmac-socfpga: complete cross-timestamp on ATSNS

[Zxyan Zhu <zxyan0222@gmail.com>]

The Agilex5 smtg_crosststamp() handler arms an internal auxiliary
snapshot, toggles GPO0 and then polls XGMAC_INT_STATUS for TSIS to
learn that the snapshot is ready.  TSIS is a transient, read-to-clear
status bit: it is set by any MAC timestamp event and cleared the moment
XGMAC_TIMESTAMP_STATUS is read.

That makes the TSIS poll racy in two ways.  A stale TSIS latched by an
unrelated event satisfies the poll immediately, before the auxiliary
snapshot is latched, so the FIFO comes back empty and *device is never
written even though the call returns 0.  Conversely a concurrent reader
of XGMAC_TIMESTAMP_STATUS, such as the TX timestamp completion path, can
clear TSIS while the poll is waiting and make it time out with "Wait for
time sync operation timeout".

The auxiliary snapshot FIFO level is also reported by the ATSNS count
in XGMAC_TIMESTAMP_STATUS.  Reading XGMAC_TIMESTAMP_STATUS does not
affect ATSNS, so the destructive reads above cannot disturb it.  Poll
ATSNS instead of TSIS, and wait for the PTP_ACR_ATSFC FIFO clear to
complete first so a stale ATSNS from a previous snapshot cannot satisfy
the poll before the new snapshot is latched.

Hold aux_ts_lock across the whole sequence instead of dropping it
right after arming, so a concurrent PTP_CLK_REQ_EXTTS request cannot
set PTP_ACR_ATSFC and flush the FIFO between the poll and the drain
loop, which would leave *device filled from an empty FIFO.

Fixes: fd8c4f645496 ("net: stmmac: socfpga: Add hardware supported cross-timestamp")
Signed-off-by: Zxyan Zhu <zxyan0222@gmail.com>
---
 .../ethernet/stmicro/stmmac/dwmac-socfpga.c   | 31 ++++++++++++++-----
 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac-socfpga.c b/drivers/net/ethernet/stmicro/stmmac/dwmac-socfpga.c
index 1d7f0a57d288..c5f71bfc7cf4 100644
--- a/drivers/net/ethernet/stmicro/stmmac/dwmac-socfpga.c
+++ b/drivers/net/ethernet/stmicro/stmmac/dwmac-socfpga.c
@@ -337,8 +337,18 @@ static int smtg_crosststamp(ktime_t *device, struct system_counterval_t *system,
 	acr_value = readl(ptpaddr + PTP_ACR);
 	acr_value |= PTP_ACR_ATSFC;
 	writel(acr_value, ptpaddr + PTP_ACR);
-	/* Release the mutex */
-	mutex_unlock(&priv->aux_ts_lock);
+
+	/* Wait for the FIFO clear to complete, so the poll below only
+	 * observes snapshots latched by this trigger.
+	 */
+	ret = readl_poll_timeout(ptpaddr + PTP_ACR, acr_value,
+				 !(acr_value & PTP_ACR_ATSFC), 10, 10000);
+	if (ret) {
+		mutex_unlock(&priv->aux_ts_lock);
+		netdev_err(priv->dev, "%s: Failed to clear snapshot FIFO\n",
+			   __func__);
+		return ret;
+	}
 
 	/* Trigger Internal snapshot signal. Create a rising edge by just toggle
 	 * the GPO0 to low and back to high.
@@ -349,10 +359,16 @@ static int smtg_crosststamp(ktime_t *device, struct system_counterval_t *system,
 	gpio_value |= XGMAC_GPIO_GPO0;
 	writel(gpio_value, ioaddr + XGMAC_GPIO_STATUS);
 
-	/* Poll for time sync operation done */
-	ret = readl_poll_timeout(priv->ioaddr + XGMAC_INT_STATUS, v,
-				 (v & XGMAC_INT_TSIS), 100, 10000);
+	/* Wait for the auxiliary snapshot to be latched: the ATSNS count
+	 * is the FIFO level and is not affected by reading
+	 * XGMAC_TIMESTAMP_STATUS, so concurrent readers cannot disturb
+	 * the poll.
+	 */
+	ret = readl_poll_timeout(ioaddr + XGMAC_TIMESTAMP_STATUS, v,
+				 FIELD_GET(XGMAC_TIMESTAMP_ATSNS_MASK, v),
+				 100, 10000);
 	if (ret) {
+		mutex_unlock(&priv->aux_ts_lock);
 		netdev_err(priv->dev, "%s: Wait for time sync operation timeout\n",
 			   __func__);
 		return ret;
@@ -364,8 +380,7 @@ static int smtg_crosststamp(ktime_t *device, struct system_counterval_t *system,
 		.use_nsecs = false,
 	};
 
-	num_snapshot = FIELD_GET(XGMAC_TIMESTAMP_ATSNS_MASK,
-				 readl(ioaddr + XGMAC_TIMESTAMP_STATUS));
+	num_snapshot = FIELD_GET(XGMAC_TIMESTAMP_ATSNS_MASK, v);
 
 	/* Repeat until the timestamps are from the FIFO last segment */
 	for (i = 0; i < num_snapshot; i++) {
@@ -375,6 +390,8 @@ static int smtg_crosststamp(ktime_t *device, struct system_counterval_t *system,
 		read_unlock_irqrestore(&priv->ptp_lock, flags);
 	}
 
+	mutex_unlock(&priv->aux_ts_lock);
+
 	get_smtgtime(priv->mii, SMTG_MDIO_ADDR, &smtg_time);
 	system->cycles = smtg_time;
 
-- 
2.34.1