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

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

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