Cortex-M NVIC can have a different number of priority bits.
Cortex-M0/M0+/M1 devices must use 2 or more bits, while devices based
on ARMv7m and up must use 3 or more bits.

This adds a "num-prio-bits" property which will get sensible default
values if unset (2 or 8 depending on the device). Unless a SOC
specifies the number of bits to use, the previous behavior is
maintained for backward compatibility.

Signed-off-by: Samuel Tardieu <sam@rfc1149.net>
Suggested-by: Anton Kochkov <anton.kochkov@proton.me>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1122
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
---
 hw/intc/armv7m_nvic.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/hw/intc/armv7m_nvic.c b/hw/intc/armv7m_nvic.c
index 50f9a973a2..404a445138 100644
--- a/hw/intc/armv7m_nvic.c
+++ b/hw/intc/armv7m_nvic.c
@@ -2572,6 +2572,11 @@ static const VMStateDescription vmstate_nvic = {
 static Property props_nvic[] = {
     /* Number of external IRQ lines (so excluding the 16 internal exceptions) */
     DEFINE_PROP_UINT32("num-irq", NVICState, num_irq, 64),
+    /*
+     * Number of the maximum priority bits that can be used. 0 means
+     * to use a reasonable default.
+     */
+    DEFINE_PROP_UINT8("num-prio-bits", NVICState, num_prio_bits, 0),
     DEFINE_PROP_END_OF_LIST()
 };
 
@@ -2685,7 +2690,23 @@ static void armv7m_nvic_realize(DeviceState *dev, Error **errp)
     /* include space for internal exception vectors */
     s->num_irq += NVIC_FIRST_IRQ;
 
-    s->num_prio_bits = arm_feature(&s->cpu->env, ARM_FEATURE_V7) ? 8 : 2;
+    if (s->num_prio_bits == 0) {
+        /*
+         * If left unspecified, use 2 bits by default on Cortex-M0/M0+/M1
+         * and 8 bits otherwise.
+         */
+        s->num_prio_bits = arm_feature(&s->cpu->env, ARM_FEATURE_V7) ? 8 : 2;
+    } else {
+        uint8_t min_prio_bits =
+            arm_feature(&s->cpu->env, ARM_FEATURE_V7) ? 3 : 2;
+        if (s->num_prio_bits < min_prio_bits || s->num_prio_bits > 8) {
+            error_setg(errp,
+                       "num-prio-bits %d is outside "
+                       "NVIC acceptable range [%d-8]",
+                       s->num_prio_bits, min_prio_bits);
+            return;
+        }
+    }
 
     /*
      * This device provides a single memory region which covers the
-- 
2.42.0