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Re: [Qemu-arm] [PATCH v2] hw/ptimer: Don't wrap around counter for expir

From: Peter Maydell
Subject: Re: [Qemu-arm] [PATCH v2] hw/ptimer: Don't wrap around counter for expired timer that uses tick handler
Date: Mon, 4 Jul 2016 10:55:07 +0100

On 1 July 2016 at 18:49, Dmitry Osipenko <address@hidden> wrote:
> On 01.07.2016 19:36, Peter Maydell wrote:
>> On 30 June 2016 at 20:01, Dmitry Osipenko <address@hidden> wrote:
>>> On 30.06.2016 18:02, Peter Maydell wrote:
>>>> What I meant was: ptimer_get_count() is typically called to generate
>>>> a value to return from a register. That's a separate thing, conceptually,
>>>> from whether the device happens to also trigger an interrupt on timer
>>>> expiry by passing a bh to ptimer_init(). So it's very odd for a detail
>>>> of interrupt-on-timer-expiry (that there is a bottom half) to affect
>>>> the value returned when you read the timer count register.
>>> In order to handle wraparound correctly, software needs to track the moment 
>>> of
>>> the wraparound - the interrupt. If software reads wrapped around counter 
>>> value
>>> before IRQ triggered (ptimer expired), then it would assume that no 
>>> wraparound
>>> happened and won't perform counter value correction, resulting in periodic
>>> counter "jumping" backwards.
>> That just says you need particular behaviour between counter reads
>> and IRQ triggers; it doesn't say that you need the behaviour to be
>> different if the ptimer code doesn't know about the IRQ trigger.
> Okay, I already explained the reason for having two different behaviours - to
> make polled counter value more distributed when possible. If I understand you
> correctly, you don't like it because it is "odd" and I agree that it's a bit 
> clumsy.

> So, what we are going to do now? Would you just revert the offending commit or
> you have some other suggestions?

Well, we need to fix the regression, but basically I'm kind of
confused at the moment. I haven't invested a lot of time in
trying to understand the timer code, so all I can really do
is say "this does not look like the right thing" and ask you
to come up with a different fix for it.

-- PMM

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