[PATCH] clocksource/drivers/riscv: Refuse to probe on T-Head

[Palmer Dabbelt <palmer@rivosinc.com>]

From: Palmer Dabbelt <palmer@rivosinc.com>

As of d9f15a9de44a ("Revert "clocksource/drivers/riscv: Events are
stopped during CPU suspend"") this driver no longer functions correctly
for the T-Head firmware.  That shouldn't impact any users, as we've got
a functioning driver that's higher priority, but let's just be safe and
ban it from probing at all.

Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
---
This feel super ugly to me, but I'm not sure how to do this more
cleanly.  I'm not even sure if it's necessary, but I just ran back into
the driver reviewing some other patches so I figured I'd say something.
---
 drivers/clocksource/timer-riscv.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
index a0d66fabf073..d2d0236d1ae6 100644
--- a/drivers/clocksource/timer-riscv.c
+++ b/drivers/clocksource/timer-riscv.c
@@ -139,6 +139,22 @@ static int __init riscv_timer_init_dt(struct device_node *n)
 	if (cpuid != smp_processor_id())
 		return 0;
 
+	/*
+	 * The T-Head firmware does not route timer interrups to the core
+	 * during non-retentive suspend.  This is allowed by the specifications
+	 * (no interrupts are required to wake up the core during non-retentive
+	 * suspend), but most systems don't behave that way and Linux just
+	 * assumes that interrupts work.
+	 *
+	 * There's another timer for the T-Head sytems that behave this way
+	 * that is already probed by default, but just to be sure skip
+	 * initializing the SBI driver as it'll just break things later.
+	 */
+	if (sbi_get_mvendorid() == THEAD_VENDOR_ID) {
+		pr_debug_once("Skipping SBI timer on T-Head due to missed wakeups");
+		return 0;
+	}
+
 	domain = NULL;
 	child = of_get_compatible_child(n, "riscv,cpu-intc");
 	if (!child) {
-- 
2.39.1