x86: timer: use a timer base of 0

On x86 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.

Change the default to not read the timer base, leaving it at 0. Add an
option for when U-Boot is the secondary bootloader.

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

arch/x86/cpu/coreboot/Kconfig

@@ -24,5 +24,6 @@ config SYS_COREBOOT
 	imply CMD_CBFS
 	imply FS_CBFS
 	imply CBMEM_CONSOLE
+	imply X86_TSC_READ_BASE
 
 endif

arch/x86/cpu/slimbootloader/Kconfig

@@ -17,3 +17,4 @@ config SYS_SLIMBOOTLOADER
 	imply USB_EHCI_HCD
 	imply USB_XHCI_HCD
 	imply E1000
+	imply X86_TSC_READ_BASE

drivers/timer/Kconfig

@@ -174,6 +174,20 @@ config X86_TSC_TIMER
 	help
 	  Select this to enable Time-Stamp Counter (TSC) timer for x86.
 
+config X86_TSC_READ_BASE
+	bool "Read the TSC timer base on start-up"
+	depends on X86_TSC_TIMER
+	help
+	  On x86 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.
+
+	  The only exception is when U-Boot is used as a secondary bootloader,
+	  where this option should be enabled.
+
 config MTK_TIMER
 	bool "MediaTek timer support"
 	depends on TIMER

drivers/timer/tsc_timer.c

@@ -397,7 +397,8 @@ 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_READ_BASE))
+		gd->arch.tsc_base = rdtsc();
 
 	if (!gd->arch.clock_rate) {
 		unsigned long fast_calibrate;

lib/efi/Kconfig

@@ -1,6 +1,7 @@
 config EFI
 	bool "Support running U-Boot from EFI"
 	depends on X86
+	imply X86_TSC_READ_BASE
 	help
 	  U-Boot can be started from EFI on certain platforms. This allows
 	  EFI to perform most of the system init and then jump to U-Boot for