ptp: ocp: Improve PCIe delay estimation

The PCIe bus can be pretty busy during boot and probe function can
see excessive delays. Let's find the minimal value out of several
tests and use it as estimated value.

Signed-off-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20240905140028.560454-1-vadim.fedorenko@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Vadim Fedorenko

drivers/ptp/ptp_ocp.c

@@ -1558,22 +1558,24 @@ ptp_ocp_watchdog(struct timer_list *t)
 static void
 ptp_ocp_estimate_pci_timing(struct ptp_ocp *bp)
 {
-	ktime_t start, end;
-	ktime_t delay;
+	ktime_t start, end, delay = U64_MAX;
 	u32 ctrl;
+	int i;
 
-	ctrl = ioread32(&bp->reg->ctrl);
-	ctrl = OCP_CTRL_READ_TIME_REQ | OCP_CTRL_ENABLE;
+	for (i = 0; i < 3; i++) {
+		ctrl = ioread32(&bp->reg->ctrl);
+		ctrl = OCP_CTRL_READ_TIME_REQ | OCP_CTRL_ENABLE;
 
-	iowrite32(ctrl, &bp->reg->ctrl);
+		iowrite32(ctrl, &bp->reg->ctrl);
 
-	start = ktime_get_ns();
+		start = ktime_get_raw_ns();
 
-	ctrl = ioread32(&bp->reg->ctrl);
+		ctrl = ioread32(&bp->reg->ctrl);
 
-	end = ktime_get_ns();
+		end = ktime_get_raw_ns();
 
-	delay = end - start;
+		delay = min(delay, end - start);
+	}
 	bp->ts_window_adjust = (delay >> 5) * 3;
 }