Re: [PATCH 7/7] hw/watchdog/wdt_aspeed: Reduce timer precision to micro-second

[Andrew Jeffery]

On Mon, 22 Jun 2020, at 18:13, Philippe Mathieu-Daudé wrote:
> On 6/22/20 2:21 AM, Andrew Jeffery wrote:
> > On Wed, 17 Jun 2020, at 13:11, Philippe Mathieu-Daudé wrote:
> >> Hi Andrew,
> >>
> >> On 6/17/20 3:18 AM, Andrew Jeffery wrote:
> >>> On Tue, 16 Jun 2020, at 17:21, Philippe Mathieu-Daudé wrote:
> >>>> The current implementation uses nano-second precision, while
> >>>> the watchdog can not be more precise than a micro-second.
> >>>
> >>> What's the basis for this assertion? It's true for the AST2500 and 
> >>> AST2600, but 
> >>> the AST2400 can run the watchdog from either a 1MHz clock source or the 
> >>> APB 
> >>> clock (which must be at least 16.5MHz on palmetto). The reset state on the
> >>> AST2400 configures the watchdog for the APB clock rate.
> >>>
> >>> The Linux driver will eventually configure the watchdog for 1MHz mode
> >>> regardless so perhaps the AST2400 reset state is a bit of a corner case, 
> >>> but
> >>> I feel the assertion should be watered down a bit?
> >>
> >> What about this description?
> >>
> >> "The current implementation uses nano-second precision, but
> >>  is not more precise than micro-second precision.
> >>  Simplify by using a micro-second based timer.
> >>  Rename the timer 'timer_us' to have the unit explicit."
> > 
> > So is this a limitation of QEMUTimer? I was establishing that the hardware 
> > can 
> > operate at greater than 1 micro-second precision.
> 
> No, I misread your comment about the AST2400 timer which can run
> at more than 1Mhz.
> 
> The QEMUTimer doesn't have a such limitation; this patch
> aimed to simplify the code for reviewers, but you proved
> it incorrect, so let's disregard it.
> 
> Thanks for your careful review!

Ah, great, I was wondering where my misunderstanding was.

Thanks for clearing that up.

Andrew