target/arm: Refactor default generic timer frequency handling

The generic timer frequency is settable by board code via a QOM
property "cntfrq", but otherwise defaults to 62.5MHz.  The way this
is done includes some complication resulting from how this was
originally a fixed value with no QOM property.  Clean it up:

 * always set cpu->gt_cntfrq_hz to some sensible value, whether
   the CPU has the generic timer or not, and whether it's system
   or user-only emulation
 * this means we can always use gt_cntfrq_hz, and never need
   the old GTIMER_SCALE define
 * set the default value in exactly one place, in the realize fn

The aim here is to pave the way for handling the ARMv8.6 requirement
that the generic timer frequency is always 1GHz.  We're going to do
that by having old CPU types keep their legacy-in-QEMU behaviour and
having the default for any new CPU types be a 1GHz rather han 62.5MHz
cntfrq, so we want the point where the default is decided to be in
one place, and in code, not in a DEFINE_PROP_UINT64() initializer.

This commit should have no behavioural changes.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20240426122913.3427983-2-peter.maydell@linaro.org

3 files changed, 29 insertions, 25 deletions

diff --git a/target/arm/cpu.c b/target/arm/cpu.c
index a152def..9f2ca66 100644
--- a/target/arm/cpu.c
+++ b/target/arm/cpu.c
@@ -1506,9 +1506,12 @@ static void arm_cpu_initfn(Object *obj)
     }
 }
 
+/*
+ * 0 means "unset, use the default value". That default might vary depending
+ * on the CPU type, and is set in the realize fn.
+ */
 static Property arm_cpu_gt_cntfrq_property =
-            DEFINE_PROP_UINT64("cntfrq", ARMCPU, gt_cntfrq_hz,
-                               NANOSECONDS_PER_SECOND / GTIMER_SCALE);
+            DEFINE_PROP_UINT64("cntfrq", ARMCPU, gt_cntfrq_hz, 0);
 
 static Property arm_cpu_reset_cbar_property =
             DEFINE_PROP_UINT64("reset-cbar", ARMCPU, reset_cbar, 0);
@@ -1954,6 +1957,17 @@ static void arm_cpu_realizefn(DeviceState *dev, Error **errp)
         return;
     }
 
+    if (!cpu->gt_cntfrq_hz) {
+        /*
+         * 0 means "the board didn't set a value, use the default".
+         * The default value of the generic timer frequency (as seen in
+         * CNTFRQ_EL0) is 62.5MHz, which corresponds to a period of 16ns.
+         * This is what you get (a) for a CONFIG_USER_ONLY CPU (b) if the
+         * board doesn't set it.
+         */
+        cpu->gt_cntfrq_hz = GTIMER_DEFAULT_HZ;
+    }
+
 #ifndef CONFIG_USER_ONLY
     /* The NVIC and M-profile CPU are two halves of a single piece of
      * hardware; trying to use one without the other is a command line
@@ -2002,18 +2016,7 @@ static void arm_cpu_realizefn(DeviceState *dev, Error **errp)
     }
 
     {
-        uint64_t scale;
-
-        if (arm_feature(env, ARM_FEATURE_GENERIC_TIMER)) {
-            if (!cpu->gt_cntfrq_hz) {
-                error_setg(errp, "Invalid CNTFRQ: %"PRId64"Hz",
-                           cpu->gt_cntfrq_hz);
-                return;
-            }
-            scale = gt_cntfrq_period_ns(cpu);
-        } else {
-            scale = GTIMER_SCALE;
-        }
+        uint64_t scale = gt_cntfrq_period_ns(cpu);
 
         cpu->gt_timer[GTIMER_PHYS] = timer_new(QEMU_CLOCK_VIRTUAL, scale,
                                                arm_gt_ptimer_cb, cpu);
diff --git a/target/arm/helper.c b/target/arm/helper.c
index bb0e1ba..7587635 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -2474,6 +2474,13 @@ static const ARMCPRegInfo v6k_cp_reginfo[] = {
       .resetvalue = 0 },
 };
 
+static void arm_gt_cntfrq_reset(CPUARMState *env, const ARMCPRegInfo *opaque)
+{
+    ARMCPU *cpu = env_archcpu(env);
+
+    cpu->env.cp15.c14_cntfrq = cpu->gt_cntfrq_hz;
+}
+
 #ifndef CONFIG_USER_ONLY
 
 static CPAccessResult gt_cntfrq_access(CPUARMState *env, const ARMCPRegInfo *ri,
@@ -3228,13 +3235,6 @@ void arm_gt_hvtimer_cb(void *opaque)
     gt_recalc_timer(cpu, GTIMER_HYPVIRT);
 }
 
-static void arm_gt_cntfrq_reset(CPUARMState *env, const ARMCPRegInfo *opaque)
-{
-    ARMCPU *cpu = env_archcpu(env);
-
-    cpu->env.cp15.c14_cntfrq = cpu->gt_cntfrq_hz;
-}
-
 static const ARMCPRegInfo generic_timer_cp_reginfo[] = {
     /*
      * Note that CNTFRQ is purely reads-as-written for the benefit
@@ -3514,7 +3514,7 @@ static const ARMCPRegInfo generic_timer_cp_reginfo[] = {
       .opc0 = 3, .opc1 = 3, .crn = 14, .crm = 0, .opc2 = 0,
       .type = ARM_CP_CONST, .access = PL0_R /* no PL1_RW in linux-user */,
       .fieldoffset = offsetof(CPUARMState, cp15.c14_cntfrq),
-      .resetvalue = NANOSECONDS_PER_SECOND / GTIMER_SCALE,
+      .resetfn = arm_gt_cntfrq_reset,
     },
     { .name = "CNTVCT_EL0", .state = ARM_CP_STATE_AA64,
       .opc0 = 3, .opc1 = 3, .crn = 14, .crm = 0, .opc2 = 2,
diff --git a/target/arm/internals.h b/target/arm/internals.h
index e40ec45..b6c78db 100644
--- a/target/arm/internals.h
+++ b/target/arm/internals.h
@@ -60,10 +60,11 @@ static inline bool excp_is_internal(int excp)
         || excp == EXCP_SEMIHOST;
 }
 
-/* Scale factor for generic timers, ie number of ns per tick.
- * This gives a 62.5MHz timer.
+/*
+ * Default frequency for the generic timer, in Hz.
+ * This is 62.5MHz, which gives a 16 ns tick period.
  */
-#define GTIMER_SCALE 16
+#define GTIMER_DEFAULT_HZ 62500000
 
 /* Bit definitions for the v7M CONTROL register */
 FIELD(V7M_CONTROL, NPRIV, 0, 1)