The IBM PPE42 processors support a 32-bit decrementer
that can raise an external interrupt when DEC[0]
transitions from a 0 to a 1 (a non-negative value to a
negative value).  It also continues decrementing
even after this condition is met.

The BookE timer is slightly different in that it
raises an interrupt when the DEC value reaches 0
and stops decrementing at that point.

Support a PPE42 version of the BookE timer by
adding a new PPC_TIMER_PPE flag that has the timer
code look for the transition from a non-negative value
to a negative value and allows the value to
continue decrementing.

Signed-off-by: Glenn Miles <milesg@linux.ibm.com>
---

Changes from v2:
  - Split out PPE42 decrementer support from v2 patch 3

 hw/ppc/ppc_booke.c   | 7 ++++++-
 include/hw/ppc/ppc.h | 1 +
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/hw/ppc/ppc_booke.c b/hw/ppc/ppc_booke.c
index 3872ae2822..13403a56b1 100644
--- a/hw/ppc/ppc_booke.c
+++ b/hw/ppc/ppc_booke.c
@@ -352,7 +352,12 @@ void ppc_booke_timers_init(PowerPCCPU *cpu, uint32_t freq, uint32_t flags)
     booke_timer = g_new0(booke_timer_t, 1);
 
     cpu->env.tb_env = tb_env;
-    tb_env->flags = flags | PPC_TIMER_BOOKE | PPC_DECR_ZERO_TRIGGERED;
+    if (flags & PPC_TIMER_PPE) {
+        /* PPE's use a modified version of the booke behavior */
+        tb_env->flags = flags | PPC_DECR_UNDERFLOW_TRIGGERED;
+    } else {
+        tb_env->flags = flags | PPC_TIMER_BOOKE | PPC_DECR_ZERO_TRIGGERED;
+    }
 
     tb_env->tb_freq    = freq;
     tb_env->decr_freq  = freq;
diff --git a/include/hw/ppc/ppc.h b/include/hw/ppc/ppc.h
index 8a14d623f8..cb51d704c6 100644
--- a/include/hw/ppc/ppc.h
+++ b/include/hw/ppc/ppc.h
@@ -52,6 +52,7 @@ struct ppc_tb_t {
 #define PPC_DECR_UNDERFLOW_LEVEL     (1 << 4) /* Decr interrupt active when
                                                * the most significant bit is 1.
                                                */
+#define PPC_TIMER_PPE                (1 << 5) /* Enable PPE support */
 
 uint64_t cpu_ppc_get_tb(ppc_tb_t *tb_env, uint64_t vmclk, int64_t tb_offset);
 void cpu_ppc_tb_init(CPUPPCState *env, uint32_t freq);
-- 
2.43.0