[PATCH v2] 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.

Use gd->arch.timer_rate_hz to override the queried value if set. This
setting needs to be done in the board file, preferrably in timer_init().

Signed-off-by: Kaustabh Chakraborty <kauschluss at disroot.org>
---
Changes in v2:
- Switched to using gd->arch.timer_rate_hz instead of a new config
  option (Ahmad Fatoum)
- Link to v1: https://lore.kernel.org/r/20251014-armv8-broken-cntfrq-v1-1-b63f9c69ffcb@disroot.org
---
 arch/arm/cpu/armv8/generic_timer.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm/cpu/armv8/generic_timer.c b/arch/arm/cpu/armv8/generic_timer.c
index 1de7ec596fc7cbbc3e78a241f163bc0a4fcad6b6..1bc72ed26ca31af8e8e72008ed7885cbde23b076 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 (gd->arch.timer_rate_hz)
+		return gd->arch.timer_rate_hz;
+
 	asm volatile("mrs %0, cntfrq_el0" : "=r" (cntfrq));
 	return cntfrq;
 }

---
base-commit: 582a04763aa80738c1c8ac60c47d1a5159a42833
change-id: 20251014-armv8-broken-cntfrq-f350f68fbf6e

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