Re: [PATCH 1/5] drm: Add drm_fixed_16_16 helper

From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Cc: dri-devel@lists.freedesktop.org,
	"Neil Armstrong" <narmstrong@baylibre.com>,
	"David Airlie" <airlied@linux.ie>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"Kevin Hilman" <khilman@baylibre.com>,
	"Jerome Brunet" <jbrunet@baylibre.com>,
	"Martin Blumenstingl" <martin.blumenstingl@googlemail.com>,
	"Rob Clark" <robdclark@gmail.com>, "Sean Paul" <sean@poorly.run>,
	"Sandy Huang" <hjc@rock-chips.com>,
	"Heiko Stübner" <heiko@sntech.de>,
	"Maarten Lankhorst" <maarten.lankhorst@linux.intel.com>,
	"Maxime Ripard" <mripard@kernel.org>,
	"Thomas Zimmermann" <tzimmermann@suse.de>,
	"Abhinav Kumar" <abhinavk@codeaurora.org>,
	"Dmitry Baryshkov" <dmitry.baryshkov@linaro.org>,
	"Lee Jones" <lee.jones@linaro.org>,
	"Stephen Boyd" <swboyd@chromium.org>,
	"Kalyan Thota" <kalyan_t@codeaurora.org>,
	"Ville Syrjälä" <ville.syrjala@linux.intel.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/5] drm: Add drm_fixed_16_16 helper
Date: Wed, 1 Sep 2021 21:13:04 +0300	[thread overview]
Message-ID: <YS/CsCSqKeFYF9x7@pendragon.ideasonboard.com> (raw)
In-Reply-To: <20210901175431.14060-1-alyssa@rosenzweig.io>

Hi Alyssa,

Thank you for the patch.

On Wed, Sep 01, 2021 at 01:54:27PM -0400, Alyssa Rosenzweig wrote:
> This constructs a fixed 16.16 rational, useful to specify the minimum
> and maximum scaling in drm_atomic_helper_check_plane_state. It is
> open-coded as a macro in multiple drivers, so let's share the helper.
> 
> Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
> ---
>  include/drm/drm_fixed.h | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/include/drm/drm_fixed.h b/include/drm/drm_fixed.h
> index 553210c02ee0..df1f369b4918 100644
> --- a/include/drm/drm_fixed.h
> +++ b/include/drm/drm_fixed.h
> @@ -208,4 +208,9 @@ static inline s64 drm_fixp_exp(s64 x)
>  	return sum;
>  }
>  

Missing documentation :-)

> +static inline int drm_fixed_16_16(s32 mult, s32 div)

You should return a s32.

The function name isn't very explicit, and departs from the naming
scheme of other functions in the same file. As fixed-point numbers are
stored in a s64 for the drm_fixp_* helpers, we shouldn't rese the
drm_fixp_ prefix, maybe drm_fixp_s16_16_ would be a good prefix. The
function should probably be named drm_fixp_s16_16 from_fraction() then,
but then the same logic should possibly be replicated to ensure optimal
precision. I wonder if it wouldn't be best to simply use
drm_fixp_from_fraction() and shift the result right by 16 bits.

> +{
> +	return (mult << 16) / div;
> +}
> +
>  #endif

-- 
Regards,

Laurent Pinchart

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