3 files changed, 57 insertions, 0 deletions

diff --git a/hw/core/ptimer.c b/hw/core/ptimer.c
index 2aa97cb665..6ba19fd965 100644
--- a/hw/core/ptimer.c
+++ b/hw/core/ptimer.c
@@ -15,6 +15,7 @@
 #include "sysemu/qtest.h"
 #include "block/aio.h"
 #include "sysemu/cpus.h"
+#include "hw/clock.h"
 
 #define DELTA_ADJUST     1
 #define DELTA_NO_ADJUST -1
@@ -348,6 +349,39 @@ void ptimer_set_period(ptimer_state *s, int64_t period)
     }
 }
 
+/* Set counter increment interval from a Clock */
+void ptimer_set_period_from_clock(ptimer_state *s, const Clock *clk,
+                                  unsigned int divisor)
+{
+    /*
+     * The raw clock period is a 64-bit value in units of 2^-32 ns;
+     * put another way it's a 32.32 fixed-point ns value. Our internal
+     * representation of the period is 64.32 fixed point ns, so
+     * the conversion is simple.
+     */
+    uint64_t raw_period = clock_get(clk);
+    uint64_t period_frac;
+
+    assert(s->in_transaction);
+    s->delta = ptimer_get_count(s);
+    s->period = extract64(raw_period, 32, 32);
+    period_frac = extract64(raw_period, 0, 32);
+    /*
+     * divisor specifies a possible frequency divisor between the
+     * clock and the timer, so it is a multiplier on the period.
+     * We do the multiply after splitting the raw period out into
+     * period and frac to avoid having to do a 32*64->96 multiply.
+     */
+    s->period *= divisor;
+    period_frac *= divisor;
+    s->period += extract64(period_frac, 32, 32);
+    s->period_frac = (uint32_t)period_frac;
+
+    if (s->enabled) {
+        s->need_reload = true;
+    }
+}
+
 /* Set counter frequency in Hz.  */
 void ptimer_set_freq(ptimer_state *s, uint32_t freq)
 {
diff --git a/include/hw/ptimer.h b/include/hw/ptimer.h
index 412763fffb..c443218475 100644
--- a/include/hw/ptimer.h
+++ b/include/hw/ptimer.h
@@ -166,6 +166,28 @@ void ptimer_transaction_commit(ptimer_state *s);
 void ptimer_set_period(ptimer_state *s, int64_t period);
 
 /**
+ * ptimer_set_period_from_clock - Set counter increment from a Clock
+ * @s: ptimer to configure
+ * @clk: pointer to Clock object to take period from
+ * @divisor: value to scale the clock frequency down by
+ *
+ * If the ptimer is being driven from a Clock, this is the preferred
+ * way to tell the ptimer about the period, because it avoids any
+ * possible rounding errors that might happen if the internal
+ * representation of the Clock period was converted to either a period
+ * in ns or a frequency in Hz.
+ *
+ * If the ptimer should run at the same frequency as the clock,
+ * pass 1 as the @divisor; if the ptimer should run at half the
+ * frequency, pass 2, and so on.
+ *
+ * This function will assert if it is called outside a
+ * ptimer_transaction_begin/commit block.
+ */
+void ptimer_set_period_from_clock(ptimer_state *s, const Clock *clock,
+                                  unsigned int divisor);
+
+/**
  * ptimer_set_freq - Set counter frequency in Hz
  * @s: ptimer to configure
  * @freq: counter frequency in Hz
diff --git a/include/qemu/typedefs.h b/include/qemu/typedefs.h
index 976b529dfb..68deb74ef6 100644
--- a/include/qemu/typedefs.h
+++ b/include/qemu/typedefs.h
@@ -34,6 +34,7 @@ typedef struct BlockDriverState BlockDriverState;
 typedef struct BusClass BusClass;
 typedef struct BusState BusState;
 typedef struct Chardev Chardev;
+typedef struct Clock Clock;
 typedef struct CompatProperty CompatProperty;
 typedef struct CoMutex CoMutex;
 typedef struct CPUAddressSpace CPUAddressSpace;