Re: [PATCH v1 1/2] clocksource/drivers/arm_global_timer: Fix maximum prescaler value

[Daniel Lezcano]

On 19/02/2024 00:18, Martin Blumenstingl wrote:
> Hi Daniel,
>
> On Sun, Feb 18, 2024 at 11:59 PM Daniel Lezcano
> <daniel.lezcano@xxxxxxxxxx> wrote:
> [...]
> > >    #define GT_CONTROL_PRESCALER_SHIFT      8
> > > -#define GT_CONTROL_PRESCALER_MAX        0xF
> > > +#define GT_CONTROL_PRESCALER_MAX        0xFF
> > >    #define GT_CONTROL_PRESCALER_MASK       (GT_CONTROL_PRESCALER_MAX << \
> > >                                         GT_CONTROL_PRESCALER_SHIFT
> >
> > Good catch!
> >
> > IMO the initial confusion is coming from the shift and the mask size.
> >
> > But should GT_CONTROL_PRESCALER_MAX be 256 ? so (0xFF + 1)
> It depends on what we consider "max" to be:
> - the register value
> - the actual number that's used in the calculation formula
>
> If we ignore the usage of GT_CONTROL_PRESCALER_MAX within
> GT_CONTROL_PRESCALER_MASK then there's only one occurrence left, which
> decrements the calculated value right before comparing it against
> GT_CONTROL_PRESCALER_MAX.
> This means: the remaining driver currently considers
> GT_CONTROL_PRESCALER_MAX to be the maximum value that can be written
> to the register, having converted the value from the calculation
> formula to register value beforehand.
>
> > The following may be less confusing:
> >
> > #define GT_CONTROL_PRESCALER_SHIFT      8
> > #define GT_CONTROL_PRESCALER_MASK       GENMASK(15,8)
> > #define GT_CONTROL_PRESCALER_MAX        (GT_CONTROL_PRESCALER_MASK >> \
> >                                           GT_CONTROL_PRESCALER_SHIFT) + 1
> If you're interested then I'll work on a follow-up patch to clean up
> the prescaler macros (using FIELD_PREP, FIELD_GET and GENMASK would
> simplify things IMO).

Yes, cleanups are welcome

> I think that this patch is still good as-is since it's small and can
> be backported easily (if someone wants to do that).

Ok, I'm fine with that


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