Re: [PATCH v3 02/13] drm/panfrost: Prevent division by 0

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [Medium] Multiplication of `status->busy_time` by 100 overflows the 32-bit `unsigned long` type on 32-bit architectures, leading to completely incorrect utilization percentages in the debug output.
- [Medium] Meaningful code and commit message mismatch regarding the clamping logic.
--

--- Patch [2]: [PATCH v3 02/13] drm/panfrost: Prevent division by 0 ---
commit 5fb0fa4b6ede9db49d32560487844d56d5235d47
Author: Adrián Larumbe <adrian.larumbe@collabora.com>

drm/panfrost: Prevent division by 0
    
When updating and debug-printing devfreq stats, in the very unlikely
offchance that total device time is less than 100 ns, clamp it to 0 to
avoid division by 0.

[Severity: Medium]
The commit message says "clamp it to 0 to avoid division by 0". Is this
description accurate? Clamping a divisor to 0 would guarantee a division
by zero, and the actual code clamps the minimum value to 1 instead of 0.

> diff --git a/drivers/gpu/drm/panfrost/panfrost_devfreq.c b/drivers/gpu/drm/panfrost/panfrost_devfreq.c
> index b51c307788110..0fe81d259274e 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_devfreq.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_devfreq.c
> @@ -78,7 +78,7 @@ static int panfrost_devfreq_get_dev_status(struct device *dev,
>  
>  	dev_dbg(pfdev->base.dev, "busy %lu total %lu %lu %% freq %lu MHz\n",
>  		status->busy_time, status->total_time,
> -		status->busy_time / (status->total_time / 100),
> +		status->busy_time * 100 / MAX(status->total_time, 1),

[Severity: Medium]
Will this multiplication overflow on 32-bit architectures?

Because `status->busy_time` is in nanoseconds, a fully busy GPU over a typical
50ms devfreq polling interval results in 50,000,000 ns. Multiplying this by 100
yields 5,000,000,000, which exceeds ULONG_MAX (4,294,967,295) on 32-bit systems
and would wrap around, reporting a roughly 14% utilization instead of 100%.

>  		status->current_frequency / 1000 / 1000);
>  
>  	return 0;

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260724-claude-fixes-v3-0-542af61abd0f@collabora.com?part=2