linux-kernel - Re: [PATCH v1 1/2] clocksource/drivers/arm_global

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Date: Mon, 19 Feb 2024 00:18:53 +0100
From: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To: Daniel Lezcano <daniel.lezcano@...aro.org>
Cc: tglx@...utronix.de, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, andrea.merello@...il.com, 
	patrice.chotard@...s.st.com, linux-amlogic@...ts.infradead.org
Subject: Re: [PATCH v1 1/2] clocksource/drivers/arm_global_timer: Fix maximum
 prescaler value

Hi Daniel,

On Sun, Feb 18, 2024 at 11:59 PM Daniel Lezcano
<daniel.lezcano@...aro.org> 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).
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).


Best regards,
Martin