[PATCH v2 2/2] hw/intc: sifive_plic: change interrupt priority register to WARL field

[Jim Shu <jim.shu@sifive.com>]

PLIC spec [1] requires interrupt source priority registers are WARL
field and the number of supported priority is power-of-2 to simplify SW
discovery.

Existing QEMU RISC-V machine (e.g. shakti_c) don't strictly follow PLIC
spec, whose number of supported priority is not power-of-2. Just change
each bit of interrupt priority register to WARL field when the number of
supported priority is power-of-2.

[1] https://github.com/riscv/riscv-plic-spec/blob/master/riscv-plic.adoc#interrupt-priorities

Signed-off-by: Jim Shu <jim.shu@sifive.com>
---
 hw/intc/sifive_plic.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/hw/intc/sifive_plic.c b/hw/intc/sifive_plic.c
index f864efa761..218ccff8bd 100644
--- a/hw/intc/sifive_plic.c
+++ b/hw/intc/sifive_plic.c
@@ -180,7 +180,15 @@ static void sifive_plic_write(void *opaque, hwaddr addr, uint64_t value,
     if (addr_between(addr, plic->priority_base, plic->num_sources << 2)) {
         uint32_t irq = ((addr - plic->priority_base) >> 2) + 1;
 
-        if (value <= plic->num_priorities) {
+        if ((plic->num_priorities + 1) & (plic->num_priorities)) {
+            /*
+             * if "num_priorities + 1" is power-of-2, make each register bit of
+             * interrupt priority WARL (Write-Any-Read-Legal). Just filter
+             * out the access to unsupported priority bits.
+             */
+            plic->source_priority[irq] = value % (plic->num_priorities + 1);
+            sifive_plic_update(plic);
+        } else if (value <= plic->num_priorities) {
             plic->source_priority[irq] = value;
             sifive_plic_update(plic);
         }
@@ -207,7 +215,16 @@ static void sifive_plic_write(void *opaque, hwaddr addr, uint64_t value,
         uint32_t contextid = (addr & (plic->context_stride - 1));
 
         if (contextid == 0) {
-            if (value <= plic->num_priorities) {
+            if ((plic->num_priorities + 1) & (plic->num_priorities)) {
+                /*
+                 * if "num_priorities + 1" is power-of-2, each register bit of
+                 * interrupt priority is WARL (Write-Any-Read-Legal). Just
+                 * filter out the access to unsupported priority bits.
+                 */
+                plic->target_priority[addrid] = value %
+                                                (plic->num_priorities + 1);
+                sifive_plic_update(plic);
+            } else if (value <= plic->num_priorities) {
                 plic->target_priority[addrid] = value;
                 sifive_plic_update(plic);
             }
-- 
2.17.1