[PATCH] armv8: implement workaround for broken CNTFRQ_EL0 value

[Kaustabh Chakraborty]

In devices where the U-Boot is used as a secondary bootloader, we rely
on the device's primary bootloader to implement CNTFRQ_EL0. However,
this reliance may lead to a non-functional timer in broken firmware.

For instance, some versions of Samsung's S-Boot don't implement it. It's
also not possible to set it in the U-Boot, because it's booted in a lower
exception level. CNTFRQ_EL0 is reported to be 0.

Thus, the only solution is to add a config option which, when enabled,
returns a hard coded value (CONFIG_COUNTER_FREQUENCY) instead of querying
CNTFRQ_EL0.

Signed-off-by: Kaustabh Chakraborty <kauschluss at disroot.org>
---
 arch/arm/cpu/armv8/Kconfig         | 9 +++++++++
 arch/arm/cpu/armv8/generic_timer.c | 4 ++++
 2 files changed, 13 insertions(+)

diff --git a/arch/arm/cpu/armv8/Kconfig b/arch/arm/cpu/armv8/Kconfig
index 199335cd6040d30a355a7fd5448a449fa7f550fb..d54d874b7caea200e1c05f522d8affe6d239a102 100644
--- a/arch/arm/cpu/armv8/Kconfig
+++ b/arch/arm/cpu/armv8/Kconfig
@@ -4,6 +4,15 @@ config CMO_BY_VA_ONLY
 	bool "Force cache maintenance to be exclusively by VA"
 	depends on !SYS_DISABLE_DCACHE_OPS
 
+config ARMV8_CNTFRQ_BROKEN
+	bool "Fix broken ARMv8 generic timer"
+	depends on COUNTER_FREQUENCY > 0
+	depends on SYS_ARCH_TIMER
+	help
+	  Say Y here if there is no trustable firmware present which sets
+	  CNTFRQ_EL0 frequency before U-Boot. U-Boot will use the
+	  CONFIG_COUNTER_FREQUENCY value as the reported frequency.
+
 config ARMV8_SPL_EXCEPTION_VECTORS
 	bool "Install crash dump exception vectors"
 	depends on SPL
diff --git a/arch/arm/cpu/armv8/generic_timer.c b/arch/arm/cpu/armv8/generic_timer.c
index 1de7ec596fc7cbbc3e78a241f163bc0a4fcad6b6..409570cf6fda79527e296b958368b552900524b1 100644
--- a/arch/arm/cpu/armv8/generic_timer.c
+++ b/arch/arm/cpu/armv8/generic_timer.c
@@ -19,6 +19,10 @@ DECLARE_GLOBAL_DATA_PTR;
 unsigned long notrace get_tbclk(void)
 {
 	unsigned long cntfrq;
+
+	if (IS_ENABLED(CONFIG_ARMV8_CNTFRQ_BROKEN))
+		return CONFIG_COUNTER_FREQUENCY;
+
 	asm volatile("mrs %0, cntfrq_el0" : "=r" (cntfrq));
 	return cntfrq;
 }

---
base-commit: a5b3be1a37e19646b9d36c91f11b23f568be4f21
change-id: 20251014-armv8-broken-cntfrq-f350f68fbf6e

Best regards,
-- 
Kaustabh Chakraborty <kauschluss at disroot.org>