Re: [Qemu-devel] [PATCH v4] timer: a9gtimer: remove loop to auto-increment comparator

[Peter Maydell]

On 8 October 2016 at 06:04, P J P <address@hidden> wrote:
> From: Prasad J Pandit <address@hidden>
>
> ARM A9MP processor has a peripheral timer with an auto-increment
> register, which holds an increment step value. A user could set
> this value to zero. When auto-increment control bit is enabled,
> it leads to an infinite loop in 'a9_gtimer_update' while
> updating comparator value. Remove this loop incrementing the
> comparator value.
>
> Reported-by: Li Qiang <address@hidden>
> Signed-off-by: Prasad J Pandit <address@hidden>
> ---
>  hw/timer/a9gtimer.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
>
> Update per
>   -> https://lists.gnu.org/archive/html/qemu-devel/2016-10/msg01393.html
>
> diff --git a/hw/timer/a9gtimer.c b/hw/timer/a9gtimer.c
> index 772f85f..02092dd 100644
> --- a/hw/timer/a9gtimer.c
> +++ b/hw/timer/a9gtimer.c
> @@ -73,6 +73,7 @@ static void a9_gtimer_update(A9GTimerState *s, bool sync)
>
>      A9GTimerUpdate update = a9_gtimer_get_update(s);
>      int i;
> +    uint64_t inc;
>      int64_t next_cdiff = 0;
>
>      for (i = 0; i < s->num_cpu; ++i) {
> @@ -82,15 +83,15 @@ static void a9_gtimer_update(A9GTimerState *s, bool sync)
>          if ((s->control & R_CONTROL_TIMER_ENABLE) &&
>                  (gtb->control & R_CONTROL_COMP_ENABLE)) {
>              /* R2p0+, where the compare function is >= */
> -            while (gtb->compare < update.new) {
> +            if (gtb->compare < update.new) {
>                  DB_PRINT("Compare event happened for CPU %d\n", i);
>                  gtb->status = 1;
> -                if (gtb->control & R_CONTROL_AUTO_INCREMENT) {
> -                    DB_PRINT("Auto incrementing timer compare by %" PRId32 
> "\n",
> -                             gtb->inc);
> -                    gtb->compare += gtb->inc;
> -                } else {
> -                    break;
> +                if (gtb->control & R_CONTROL_AUTO_INCREMENT && gtb->inc) {
> +                    inc = update.new - gtb->compare - 1;

Can you explain why the '- 1' here ?

> +                    inc = QEMU_ALIGN_DOWN(inc, gtb->inc);
> +                    DB_PRINT("Auto incrementing timer compare by %"
> +                                                        PRId64 "\n", inc);
> +                    gtb->compare += inc;

Something still doesn't look right here. Consider the case where
update.new is only just bigger than gtb->compare (actually the
usual case, I think). In this case 'update.new - gtb->compare - 1'
is smaller than gtb->inc and so the QEMU_ALIGN_DOWN will produce
an inc value of 0. That would be wrong, because we should definitely
have done an auto-increment.

>                  }
>              }
>              cdiff = (int64_t)gtb->compare - (int64_t)update.new + 1;
> --
> 2.5.5

thanks
-- PMM