linux-kernel - Re: [PATCH] drm/i915: Explicitly cast divisor to fix Coccinelle warning

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Message-ID: <Zo57j8G2AQbhpORB@intel.com>
Date: Wed, 10 Jul 2024 15:16:15 +0300
From: Ville Syrjälä <ville.syrjala@...ux.intel.com>
To: Thorsten Blum <thorsten.blum@...lux.com>
Cc: jani.nikula@...ux.intel.com, joonas.lahtinen@...ux.intel.com,
	rodrigo.vivi@...el.com, tursulin@...ulin.net, airlied@...il.com,
	daniel@...ll.ch, intel-gfx@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/i915: Explicitly cast divisor to fix Coccinelle
 warning

On Wed, Jul 10, 2024 at 01:55:32PM +0200, Thorsten Blum wrote:
> On 10. Jul 2024, at 13:38, Ville Syrjälä <ville.syrjala@...ux.intel.com> wrote:
> > On Wed, Jul 10, 2024 at 09:46:51AM +0200, Thorsten Blum wrote:
> >> 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
> >> 
> >> Signed-off-by: Thorsten Blum <thorsten.blum@...lux.com>
> >> ---
> >> drivers/gpu/drm/i915/i915_perf.c | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >> 
> >> diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> >> index 0b1cd4c7a525..24722e758aaf 100644
> >> --- a/drivers/gpu/drm/i915/i915_perf.c
> >> +++ b/drivers/gpu/drm/i915/i915_perf.c
> >> @@ -4103,7 +4103,7 @@ static int read_properties_unlocked(struct i915_perf *perf,
> >>  */
> >> if (oa_period <= NSEC_PER_SEC) {
> >> u64 tmp = NSEC_PER_SEC;
> >> - do_div(tmp, oa_period);
> >> + do_div(tmp, (u32)oa_period);
> > 
> > Why is this code even using do_div() when it doesn't need the
> > remainder?
> 
> do_div() is an optimized 64-by-32 division and the compiler should
> automatically remove the remainder if it's not used.

The point is that do_div() is a bad API because it magically
changes the divided in place. There are more sensible 64bit
division helpers in math64.h that can be used instead.

oa_exponent_to_ns() also hand rolls a DIV_ROUND_UP_ULL()
for some reason...

-- 
Ville Syrjälä
Intel