[PATCH v3 3/4] target/riscv: No need to re-start QEMU timer when timecmp == UINT64_MAX

[Anup Patel <apatel@ventanamicro.com>]

The time CSR will wrap-around immediately after reaching UINT64_MAX
so we don't need to re-start QEMU timer when timecmp == UINT64_MAX
in riscv_timer_write_timecmp().

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
---
 target/riscv/time_helper.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/target/riscv/time_helper.c b/target/riscv/time_helper.c
index 4fb2a471a9..b654f91af9 100644
--- a/target/riscv/time_helper.c
+++ b/target/riscv/time_helper.c
@@ -72,6 +72,30 @@ void riscv_timer_write_timecmp(RISCVCPU *cpu, QEMUTimer *timer,
         riscv_cpu_update_mip(cpu, timer_irq, BOOL_TO_MASK(0));
     }
 
+    /*
+     * Sstc specification says the following about timer interrupt:
+     * "A supervisor timer interrupt becomes pending - as reflected in
+     * the STIP bit in the mip and sip registers - whenever time contains
+     * a value greater than or equal to stimecmp, treating the values
+     * as unsigned integers. Writes to stimecmp are guaranteed to be
+     * reflected in STIP eventually, but not necessarily immediately.
+     * The interrupt remains posted until stimecmp becomes greater
+     * than time - typically as a result of writing stimecmp."
+     *
+     * When timecmp = UINT64_MAX, the time CSR will eventually reach
+     * timecmp value but on next timer tick the time CSR will wrap-around
+     * and become zero which is less than UINT64_MAX. Now, the timer
+     * interrupt behaves like a level triggered interrupt so it will
+     * become 1 when time = timecmp = UINT64_MAX and next timer tick
+     * it will become 0 again because time = 0 < timecmp = UINT64_MAX.
+     *
+     * Based on above, we don't re-start the QEMU timer when timecmp
+     * equals UINT64_MAX.
+     */
+    if (timecmp == UINT64_MAX) {
+        return;
+    }
+
     /* otherwise, set up the future timer interrupt */
     diff = timecmp - rtc_r;
     /* back to ns (note args switched in muldiv64) */
-- 
2.34.1