Re: [Qemu-arm] [PATCH v2] hw/ptimer: Don't wrap around counter for expired timer that uses tick handler

[Peter Maydell]

On 25 June 2016 at 13:35, Dmitry Osipenko <address@hidden> wrote:
> Software should see timer counter wrap around only after IRQ being triggered.
> Change returned counter value to "1" for the expired timer and avoid returning
> wrapped around counter value in periodic mode for the timer that has 
> bottom-half
> handler setup, assuming it drives timer IRQ.
>
> This fixes regression introduced by the commit 5a50307 ("hw/ptimer: Perform
> counter wrap around if timer already expired") on SPARC emulated machine as
> reported by Mark Cave-Ayland.
>
> Signed-off-by: Dmitry Osipenko <address@hidden>
> ---
>  hw/core/ptimer.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/hw/core/ptimer.c b/hw/core/ptimer.c
> index 05b0c27..8006442 100644
> --- a/hw/core/ptimer.c
> +++ b/hw/core/ptimer.c
> @@ -93,10 +93,10 @@ uint64_t ptimer_get_count(ptimer_state *s)
>          bool oneshot = (s->enabled == 2);
>
>          /* Figure out the current counter value.  */
> -        if (s->period == 0 || (expired && (oneshot || use_icount))) {
> +        if (expired && (oneshot || use_icount || s->bh != NULL)) {
>              /* Prevent timer underflowing if it should already have
>                 triggered.  */
> -            counter = 0;
> +            counter = 1;
>          } else {
>              uint64_t rem;
>              uint64_t div;

I guess this fixes a regression, but it looks really weird.
Why should the timer behaviour change if there happens to be
a bottom half present? That should be an internal implementation
detail. It's also a bit odd that use_icount is in the check:
that shouldn't generally affect device emulation behaviour...

thanks
-- PMM