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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: Thu, 30 Jun 2016 16:02:22 +0100

On 27 June 2016 at 19:26, Dmitry Osipenko <address@hidden> wrote:
> On 27.06.2016 16:27, Peter Maydell wrote:
>> 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...
> In case of a polled timer that doesn't have ptimer trigger bottom half 
> callback
> setup, we are free to wrap around counter since timer behaviour isn't changed
> from ptimer user perspective, as it won't be able to change it's state in the
> handler.
> I just decided to keep that wraparound feature for a case of a polled free
> running timer, this should result in a better distribution of the polled 
> value.
> The potential users of that feature are "imx_epit" and "digic" timer device
> models. I should have mentioned it in the commit message to avoid confusion, 
> sorry.
> It is still an internal implementation detail, not sure what you are meaning.
> Could you elaborate, please?

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.

-- PMM

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