hw/timer/imx_epit: Avoid assertion when CR.SWR is written

The imx_epit device has a software-controllable reset triggered by
setting the SWR bit in the CR register. An error in commit cc2722ec83ad9
means that we will end up assert()ing if the guest does this, because
the code in imx_epit_write() starts ptimer transactions, and then
imx_epit_reset() also starts ptimer transactions, triggering
"ptimer_transaction_begin: Assertion `!s->in_transaction' failed".

The cleanest way to avoid this double-transaction is to move the
start-transaction for the CR write handling down below the check of
the SWR bit.

Fixes: https://bugs.launchpad.net/qemu/+bug/1880424
Fixes: cc2722ec83ad944505fe
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20200727154550.3409-1-peter.maydell@linaro.org

1 files changed, 10 insertions, 3 deletions

diff --git a/hw/timer/imx_epit.c b/hw/timer/imx_epit.c
index baf6338e1a..ebd58254d1 100644
--- a/hw/timer/imx_epit.c
+++ b/hw/timer/imx_epit.c
@@ -199,15 +199,22 @@ static void imx_epit_write(void *opaque, hwaddr offset, uint64_t value,
 
     switch (offset >> 2) {
     case 0: /* CR */
-        ptimer_transaction_begin(s->timer_cmp);
-        ptimer_transaction_begin(s->timer_reload);
 
         oldcr = s->cr;
         s->cr = value & 0x03ffffff;
         if (s->cr & CR_SWR) {
             /* handle the reset */
             imx_epit_reset(DEVICE(s));
-        } else {
+            /*
+             * TODO: could we 'break' here? following operations appear
+             * to duplicate the work imx_epit_reset() already did.
+             */
+        }
+
+        ptimer_transaction_begin(s->timer_cmp);
+        ptimer_transaction_begin(s->timer_reload);
+
+        if (!(s->cr & CR_SWR)) {
             imx_epit_set_freq(s);
         }