[U-Boot] [PATCH v3 023/108] x86: timer: Allow a timer base of 0

[Simon Glass]

On some platforms the timer is reset to 0 when the SoC is reset. Having
this as the timer base is useful since it provides an indication of how
long it takes before U-Boot is running.

When U-Boot sets the timer base to something else, time is lost and we
no-longer have an accurate account of the time since reset. This
particularly affects bootstage.

Add an option to leave the timer base as 0 on boot.

Signed-off-by: Simon Glass <sjg at chromium.org>
---

Changes in v3: None
Changes in v2: None

 drivers/timer/Kconfig     | 13 +++++++++++++
 drivers/timer/tsc_timer.c |  3 ++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/timer/Kconfig b/drivers/timer/Kconfig
index 5f4bc6edb67..0c922f54603 100644
--- a/drivers/timer/Kconfig
+++ b/drivers/timer/Kconfig
@@ -174,6 +174,19 @@ config X86_TSC_TIMER
 	help
 	  Select this to enable Time-Stamp Counter (TSC) timer for x86.
 
+config X86_TSC_ZERO_BASE
+	bool "Assume that TSC timer starts at 0"
+	depends on X86_TSC_TIMER
+	help
+	  On some platforms the TSC timer tick starts at the value 0 on reset.
+	  This it makes no sense to read the timer on boot and use that as the
+	  base, since we will miss some time taken to load U-Boot, etc. This
+	  delay is controlled by the SoC and we cannot reduce it, but for
+	  bootstage we want to record the time since reset as accurately as
+	  possible.
+
+	  Enable this to always us an initial TSC value of 0.
+
 config MTK_TIMER
 	bool "MediaTek timer support"
 	depends on TIMER
diff --git a/drivers/timer/tsc_timer.c b/drivers/timer/tsc_timer.c
index 637c8ff25a5..ac7ba7fb00d 100644
--- a/drivers/timer/tsc_timer.c
+++ b/drivers/timer/tsc_timer.c
@@ -396,7 +396,8 @@ static void tsc_timer_ensure_setup(bool early)
 {
 	if (gd->arch.tsc_inited)
 		return;
-	gd->arch.tsc_base = rdtsc();
+	if (!IS_ENABLED(CONFIG_X86_TSC_ZERO_BASE))
+		gd->arch.tsc_base = rdtsc();
 
 	if (!gd->arch.clock_rate) {
 		unsigned long fast_calibrate;
-- 
2.23.0.866.gb869b98d4c-goog