[PATCH] clocksource/drivers/arm_arch_timer: Workaround bcm2712 broken EL2 virtual timer

[Marc Zyngier <maz@kernel.org>]

It appears that the bcm2712 SoC found in the relatively popular
RPi5 has a broken EL2 virtual timer.

We do not know the reason why the timer isn't working (the timer
is ticking, but the interrupt never fires), and the SoC vendor
doesn't communicate on the reason why this isn't working, leaving
users and maintainers in the dark.

Paper over the issue by detecting the broken HW, falling back to
the physical timer instead, and let the user know about it.
Also taint the kernel as the machine is definitely not compliant
with the spec, and we don't know what else is wrong with it.

Reported-by: John <therealgraysky@proton.me>
Reported-by: Daniel Drake <dan@reactivated.net>
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: Florian Fainelli <florian.fainelli@broadcom.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Mark Rutland <mark.rutland@arm.com>
---
 drivers/clocksource/arm_arch_timer.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
index 4adf756423de9..7b4a98df6962b 100644
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c
@@ -1090,6 +1090,27 @@ static int __init arch_timer_common_init(void)
 	return arch_timer_arch_init();
 }
 
+static bool __init has_broken_el2_vtimer(void)
+{
+	/*
+	 * SoCs described here have been found to be broken, though no
+	 * explanation has been volunteered by the vendor. Let the user know
+	 * we're papering over the vendor's lack of communication.
+	 */
+	static const char * const broken_el2_vtimer[] __initconst = {
+		"brcm,bcm2712",
+		NULL
+	};
+
+	if (of_machine_compatible_match(broken_el2_vtimer)) {
+		add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+		pr_warn_once(HW_ERR "Known broken EL2 virtual timer, ignoring it\n");
+		return true;
+	}
+
+	return false;
+}
+
 /**
  * arch_timer_select_ppi() - Select suitable PPI for the current system.
  *
@@ -1115,7 +1136,8 @@ static int __init arch_timer_common_init(void)
 static enum arch_timer_ppi_nr __init arch_timer_select_ppi(void)
 {
 	if (is_kernel_in_hyp_mode()) {
-		if (arch_timer_ppi[ARCH_TIMER_HYP_VIRT_PPI])
+		if (arch_timer_ppi[ARCH_TIMER_HYP_VIRT_PPI] &&
+		    !has_broken_el2_vtimer())
 			return ARCH_TIMER_HYP_VIRT_PPI;
 
 		pr_warn_once(FW_BUG "VHE-capable CPU without EL2 virtual timer interrupt\n");
-- 
2.47.3