riscv: clocksource: Fix stimecmp update hazard on RV32

On RV32, updating the 64-bit stimecmp (or vstimecmp) CSR requires two
separate 32-bit writes. A race condition exists if the timer triggers
during these two writes.

The RISC-V Privileged Specification (e.g., Section 3.2.1 for mtimecmp)
recommends a specific 3-step sequence to avoid spurious interrupts
when updating 64-bit comparison registers on 32-bit systems:

1. Set the low-order bits (stimecmp) to all ones (ULONG_MAX).
2. Set the high-order bits (stimecmph) to the desired value.
3. Set the low-order bits (stimecmp) to the desired value.

Current implementation writes the LSB first without ensuring a future
value, which may lead to a transient state where the 64-bit comparison
is incorrectly evaluated as "expired" by the hardware. This results in
spurious timer interrupts.

This patch adopts the spec-recommended 3-step sequence to ensure the
intermediate 64-bit state is never smaller than the current time.

Fixes: 9f7a8ff ("RISC-V: Prefer sstc extension if available")
Signed-off-by: Naohiko Shimizu <naohiko.shimizu@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://patch.msgid.link/20260104135938.524-2-naohiko.shimizu@gmail.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>

Author: pshimizu

drivers/clocksource/timer-riscv.c

@@ -50,8 +50,9 @@ static int riscv_clock_next_event(unsigned long delta,
 
 	if (static_branch_likely(&riscv_sstc_available)) {
 #if defined(CONFIG_32BIT)
-		csr_write(CSR_STIMECMP, next_tval & 0xFFFFFFFF);
+		csr_write(CSR_STIMECMP, ULONG_MAX);
 		csr_write(CSR_STIMECMPH, next_tval >> 32);
+		csr_write(CSR_STIMECMP, next_tval & 0xFFFFFFFF);
 #else
 		csr_write(CSR_STIMECMP, next_tval);
 #endif