linux-kernel - Re: [PATCH 1/5] drm: Add drm_fixed_16

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Date:   Wed, 1 Sep 2021 21:13:04 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Alyssa Rosenzweig <alyssa@...enzweig.io>
Cc:     dri-devel@...ts.freedesktop.org,
        Neil Armstrong <narmstrong@...libre.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Kevin Hilman <khilman@...libre.com>,
        Jerome Brunet <jbrunet@...libre.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>,
        Sandy Huang <hjc@...k-chips.com>,
        Heiko Stübner <heiko@...ech.de>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Abhinav Kumar <abhinavk@...eaurora.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Lee Jones <lee.jones@...aro.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Kalyan Thota <kalyan_t@...eaurora.org>,
        Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] drm: Add drm_fixed_16_16 helper

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@...enzweig.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