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

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Message-ID: <5f6229ac-1390-41b5-af91-53799641d3b2@linaro.org>
Date: Mon, 19 Feb 2024 00:48:37 +0100
From: Daniel Lezcano <daniel.lezcano@...aro.org>
To: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
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

On 19/02/2024 00:18, Martin Blumenstingl wrote:
> 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).

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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