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Message-ID: <ec0b3dfb-3ce2-43f2-9cd4-042c3aca4cf7@linaro.org>
Date: Sun, 18 Feb 2024 23:59:07 +0100
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Subject: Re: [PATCH v1 1/2] clocksource/drivers/arm_global_timer: Fix maximum
 prescaler value
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To: Martin Blumenstingl <martin.blumenstingl@googlemail.com>,
 tglx@linutronix.de
Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
 andrea.merello@gmail.com, patrice.chotard@foss.st.com,
 linux-amlogic@lists.infradead.org
References: <20240218174138.1942418-1-martin.blumenstingl@googlemail.com>
 <20240218174138.1942418-2-martin.blumenstingl@googlemail.com>
From: Daniel Lezcano <daniel.lezcano@linaro.org>
In-Reply-To: <20240218174138.1942418-2-martin.blumenstingl@googlemail.com>
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On 18/02/2024 18:41, Martin Blumenstingl wrote:
> The prescaler in the "Global Timer Control Register bit assignments" is
> documented to use bits [15:8], which means that the maximum prescaler
> register value is 0xff.
> 
> Fixes: 171b45a4a70e ("clocksource/drivers/arm_global_timer: Implement rate compensation whenever source clock changes")
> Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
> ---
>   drivers/clocksource/arm_global_timer.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/clocksource/arm_global_timer.c b/drivers/clocksource/arm_global_timer.c
> index 44a61dc6f932..e1c773bb5535 100644
> --- a/drivers/clocksource/arm_global_timer.c
> +++ b/drivers/clocksource/arm_global_timer.c
> @@ -32,7 +32,7 @@
>   #define GT_CONTROL_IRQ_ENABLE		BIT(2)	/* banked */
>   #define GT_CONTROL_AUTO_INC		BIT(3)	/* banked */
>   #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)

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


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