x86: timer: tsc: Allow specifying clock rate from device tree again

With the introduction of early timer support in the TSC driver,
the capability of getting clock rate from device tree was lost
unfortunately. Now we bring such functionality back, but with a
limitation that when TSC is used as early timer, specifying clock
rate from device tree does not work.

This fixes random boot failures seen on QEMU targets: printing "TSC
frequency is ZERO" and reset forever.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/timer/tsc_timer.c

@@ -377,14 +377,23 @@ static int tsc_timer_probe(struct udevice *dev)
 {
 	struct timer_dev_priv *uc_priv = dev_get_uclass_priv(dev);
 
-	tsc_timer_ensure_setup();
-	uc_priv->clock_rate = gd->arch.clock_rate;
+	if (!uc_priv->clock_rate) {
+		tsc_timer_ensure_setup();
+		uc_priv->clock_rate = gd->arch.clock_rate;
+	} else {
+		gd->arch.tsc_base = rdtsc();
+	}
 
 	return 0;
 }
 
 unsigned long notrace timer_early_get_rate(void)
 {
+	/*
+	 * When TSC timer is used as the early timer, be warned that the timer
+	 * clock rate can only be calibrated via some hardware ways. Specifying
+	 * it in the device tree won't work for the early timer.
+	 */
 	tsc_timer_ensure_setup();
 
 	return gd->arch.clock_rate;