Re: [Qemu-devel] [PATCH 1/2] arm_mptimer: Fix timer shutdown and mode change

[Peter Crosthwaite <peter.crosthwaite@xilinx.com>]

On Sun, Jul 5, 2015 at 8:39 AM, Dmitry Osipenko <digetx@gmail.com> wrote:
> Timer, running in periodic mode, can't be stopped or coming one-shot
> tick won't be canceled because timer control code just doesn't handle
> timer disabling. Fix it by deleting the timer if enable bit isn't set.
>
> Timer won't start periodic ticking if ONE-SHOT -> PERIODIC mode change
> happened after one-shot tick was completed. Fix it by starting ticking
> only if the timer isn't ticking right now.
>
> To avoid code churning, these two fixes are squashed in one commit.
>
> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> ---
>
> Commits are squashed as per Peter Crosthwaite suggestion.
>
>  hw/timer/arm_mptimer.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/hw/timer/arm_mptimer.c b/hw/timer/arm_mptimer.c
> index 8b93b3c..0e132b1 100644
> --- a/hw/timer/arm_mptimer.c
> +++ b/hw/timer/arm_mptimer.c
> @@ -122,11 +122,18 @@ static void timerblock_write(void *opaque, hwaddr addr,
>      case 8: /* Control.  */
>          old = tb->control;
>          tb->control = value;
> -        if (((old & 1) == 0) && (value & 1)) {
> -            if (tb->count == 0 && (tb->control & 2)) {
> +        if (value & 1) {
> +            if ((old & 1) && (tb->count != 0)) {
> +                /* Do nothing if timer is ticking right now.  */
> +                break;
> +            }
> +            if (tb->control & 2) {

So when the timer was previously disabled (!(old & 1)) and the count
is non-zero this will cause a spurious auto-reload. I don't this
causes a bug today because the code as-is doesn't support arbitrary
count values, but it is a developer trap should the assumption that
tb->count equals either 0 or the reload value not hold true.

>                  tb->count = tb->load;
>              }
>              timerblock_reload(tb, 1);
> +        } else if (old & 1) {
> +            /* Shutdown the timer.  */
> +            timer_del(tb->timer);

In general, this seems to now dup the code paths for control and
load/counter writes. Both now have a del and reload call for various
changes-of state. I had a go to see if I can consolidate. Turns out,
doing so should implement timer pause and resumption while fixing both
of your bugs, I'll send some patches.

Regards,
Peter

>          }
>          break;
>      case 12: /* Interrupt status.  */
> --
> 2.4.4
>
>