Re: [Qemu-devel] [PATCH v8 2/4] hw/ptimer: Perform tick and counter wrap around if timer already expired

[Dmitry Osipenko <digetx@gmail.com>]

06.01.2016 15:17, Peter Crosthwaite пишет:
> On Tue, Jan 05, 2016 at 05:33:27AM +0300, Dmitry Osipenko wrote:
>> ptimer_get_count() might be called while QEMU timer already been expired.
>> In that case ptimer would return counter = 0, which might be undesirable
>> in case of polled timer. Do counter wrap around for periodic timer to keep
>> it distributed.
>>
>> In addition, there is no reason to keep expired timer tick deferred, so
>> just perform the tick from ptimer_get_count().
>>
>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
>> ---
>>   hw/core/ptimer.c | 35 +++++++++++++++++++++++++++++------
>>   1 file changed, 29 insertions(+), 6 deletions(-)
>>
>> diff --git a/hw/core/ptimer.c b/hw/core/ptimer.c
>> index 035af97..96a6c7a 100644
>> --- a/hw/core/ptimer.c
>> +++ b/hw/core/ptimer.c
>> @@ -85,15 +85,21 @@ static void ptimer_tick(void *opaque)
>>
>>   uint64_t ptimer_get_count(ptimer_state *s)
>>   {
>> +    int enabled = s->enabled;
>
> Variable localisation not needed.
>
>>       int64_t now;
>> +    int64_t next;
>>       uint64_t counter;
>> +    int expired;
>> +    int oneshot;
>
> Variable defs can be localised to the if (enabled) (even though now
> in original code doesn't do that).
>

Yeah, I just tried to keep original style here.

>>
>> -    if (s->enabled) {
>> +    if (enabled) {
>>           now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
>> +        next = s->next_event;
>> +        expired = (now - next >= 0);
>> +        oneshot = (enabled == 2);
>>           /* Figure out the current counter value.  */
>
> This comment is now out of place.
>

Okay, will fix it.

>> -        if (now - s->next_event > 0
>> -            || s->period == 0) {
>> -            /* Prevent timer underflowing if it should already have
>> +        if (s->period == 0 || (expired && oneshot)) {
>> +            /* Prevent one-shot timer underflowing if it should already have
>>                  triggered.  */
>>               counter = 0;
>>           } else {
>> @@ -114,12 +120,12 @@ uint64_t ptimer_get_count(ptimer_state *s)
>>                  backwards.
>>               */
>>
>> -            if ((s->enabled == 1) && (s->limit * period < 10000)) {
>> +            if (!oneshot && (s->limit * period < 10000)) {
>>                   period = 10000 / s->limit;
>>                   period_frac = 0;
>>               }
>>
>> -            rem = s->next_event - now;
>> +            rem = expired ? now - next : next - now;
>>               div = period;
>>
>>               clz1 = clz64(rem);
>> @@ -139,6 +145,23 @@ uint64_t ptimer_get_count(ptimer_state *s)
>>                       div += 1;
>>               }
>>               counter = rem / div;
>> +
>> +            if (expired) {
>> +                /* Wrap around periodic counter.  */
>> +                counter = s->delta = s->limit - counter % s->limit;
>
> Why do you update the delta here?
>

Because we would want to schedule next tick based on current wrapped around 
counter value and not some arbitrary delta.

> Also can you just get ptimer_reload to do the modulo math for you? If the
> timer is !oneshot and expired, then you call ptimer_reload anyway,
> which will update next_event. When the expired test returns false
> you can just reliably use the original logic involving now and next.
>

Yes, that's what I changed in V9. Have you received it?

https://lists.nongnu.org/archive/html/qemu-devel/2016-01/msg00272.html

>> +            }
>> +        }
>> +
>> +        if (expired) {
>> +            if (oneshot) {
>
> This if-else has a lot of common structure with the one above. I think
> they could be merged.
>

That's a good suggestion, will do it in V10. Thanks.

> Regards,
> Peter
>
>> +                ptimer_tick(s);
>> +            } else {
>> +                /* Don't use ptimer_tick() for the periodic timer since it
>> +                 * would reset the delta value.
>> +                 */
>> +                ptimer_trigger(s);
>> +                ptimer_reload(s);
>> +            }
>>           }
>>       } else {
>>           counter = s->delta;
>> --
>> 2.6.4
>>


-- 
Dmitry