[PATCH v2] drm/i915: Explicitly cast divisor and use div_u64()

[Cavitt, Jonathan]

-----Original Message-----
From: Intel-gfx <intel-gfx-bounces at lists.freedesktop.org> On Behalf Of Thorsten Blum
Sent: Friday, August 2, 2024 9:03 AM
To: jani.nikula at linux.intel.com; joonas.lahtinen at linux.intel.com; Vivi, Rodrigo <rodrigo.vivi at intel.com>; tursulin at ursulin.net; airlied at gmail.com; daniel at ffwll.ch
Cc: intel-gfx at lists.freedesktop.org; dri-devel at lists.freedesktop.org; linux-kernel at vger.kernel.org; Thorsten Blum <thorsten.blum at toblux.com>
Subject: [PATCH v2] drm/i915: Explicitly cast divisor and use div_u64()
> 
> As the comment explains, the if check ensures that the divisor oa_period
> is a u32. Explicitly cast oa_period to u32 to remove the following
> Coccinelle/coccicheck warning reported by do_div.cocci:
> 
>   WARNING: do_div() does a 64-by-32 division, please consider using div64_u64 instead
> 
> Use the preferred div_u64() function instead of the do_div() macro and
> remove the now unnecessary local variable tmp.
> 
> Signed-off-by: Thorsten Blum <thorsten.blum at toblux.com>
> ---
> Changes in v2:
> - Use div_u64() instead of do_div() after feedback from Ville Syrjälä
> - Link to v1: https://lore.kernel.org/linux-kernel/20240710074650.419902-2-thorsten.blum@toblux.com/
> ---
>  drivers/gpu/drm/i915/i915_perf.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> index 0b1cd4c7a525..f65fbe13ab59 100644
> --- a/drivers/gpu/drm/i915/i915_perf.c
> +++ b/drivers/gpu/drm/i915/i915_perf.c
> @@ -4096,15 +4096,13 @@ static int read_properties_unlocked(struct i915_perf *perf,
>  			oa_period = oa_exponent_to_ns(perf, value);
>  
>  			/* This check is primarily to ensure that oa_period <=
> -			 * UINT32_MAX (before passing to do_div which only
> +			 * UINT32_MAX (before passing it to div_u64 which only
>  			 * accepts a u32 denominator), but we can also skip
>  			 * checking anything < 1Hz which implicitly can't be
>  			 * limited via an integer oa_max_sample_rate.
>  			 */
>  			if (oa_period <= NSEC_PER_SEC) {
> -				u64 tmp = NSEC_PER_SEC;
> -				do_div(tmp, oa_period);
> -				oa_freq_hz = tmp;
> +				oa_freq_hz = div_u64(NSEC_PER_SEC, (u32)oa_period);
>  			} else
>  				oa_freq_hz = 0;

Non-blocking suggestion: this looks like it can be inlined.  And if the
inline route is taken, it might be best to invert the conditional check
like such:

oa_freq_hz = oa_period > NSEC_PER_SEC ? 0 :
                                     div_u64(NSEC_PER_SEC, (u32)oa_period);

I think this is just a matter of preference, though.  The explicit if-else
block is definitely clearer.
Reviewed-by: Jonathan Cavitt <jonathan.cavitt at intel.com>
-Jonathan Cavitt

>  
> -- 
> 2.45.2
> 
>