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Date:   Sat, 10 Sep 2022 18:10:35 -0100
From:   Melissa Wen <mwen@igalia.com>
To:     Igor Matheus Andrade Torrente <igormtorrente@gmail.com>
Cc:     rodrigosiqueiramelo@gmail.com, melissa.srw@gmail.com,
        airlied@linux.ie, daniel@ffwll.ch,
        Sudip Mukherjee <sudipm.mukherjee@gmail.com>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] drm/vkms: fix 32bit compilation error by replacing macros
Message-ID: <20220910191035.ukxhlhbc3mscbqis@mail.igalia.com>
References: <20220909114133.267689-1-mwen@igalia.com>
 <29f87796-b288-7cdc-86dd-050cf7f0b5dd@gmail.com>
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On 09/09, Igor Matheus Andrade Torrente wrote:
> Hi Mellisa,
>=20
> Thanks for the patch fixing my mistakes.
>=20
> On 9/9/22 08:41, Melissa Wen wrote:
> > Replace vkms_formats macros for fixed-point operations with functions
> > from drm/drm_fixed.h to do the same job and fix 32-bit compilation
> > errors.
> >=20
> > Fixes: a19c2ac9858 ("drm: vkms: Add support to the RGB565 format")
> > Tested-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
> > Reported-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
> > Reported-by: kernel test robot <lkp@intel.com>
> > Signed-off-by: Melissa Wen <mwen@igalia.com>
> > ---
> >   drivers/gpu/drm/vkms/vkms_formats.c | 53 +++++++++++------------------
> >   1 file changed, 19 insertions(+), 34 deletions(-)
> >=20
> > diff --git a/drivers/gpu/drm/vkms/vkms_formats.c b/drivers/gpu/drm/vkms=
/vkms_formats.c
> > index 300abb4d1dfe..ddcd3cfeeaac 100644
> > --- a/drivers/gpu/drm/vkms/vkms_formats.c
> > +++ b/drivers/gpu/drm/vkms/vkms_formats.c
> > @@ -1,27 +1,12 @@
> >   // SPDX-License-Identifier: GPL-2.0+
> > -#include <drm/drm_rect.h>
> > +#include <linux/kernel.h>
> >   #include <linux/minmax.h>
> > +#include <drm/drm_rect.h>
> > +#include <drm/drm_fixed.h>
> >   #include "vkms_formats.h"
> > -/* The following macros help doing fixed point arithmetic. */
> > -/*
> > - * With Fixed-Point scale 15 we have 17 and 15 bits of integer and fra=
ctional
> > - * parts respectively.
> > - *  | 0000 0000 0000 0000 0.000 0000 0000 0000 |
> > - * 31                                          0
> > - */
> > -#define SHIFT 15
> > -
> > -#define INT_TO_FIXED(a) ((a) << SHIFT)
> > -#define FIXED_MUL(a, b) ((s32)(((s64)(a) * (b)) >> SHIFT))
> > -#define FIXED_DIV(a, b) ((s32)(((s64)(a) << SHIFT) / (b)))
> > -/* This macro converts a fixed point number to int, and round half up =
it */
> > -#define FIXED_TO_INT_ROUND(a) (((a) + (1 << (SHIFT - 1))) >> SHIFT)
> > -#define INT_TO_FIXED_DIV(a, b) (FIXED_DIV(INT_TO_FIXED(a), INT_TO_FIXE=
D(b)))
> > -#define INT_TO_FIXED_DIV(a, b) (FIXED_DIV(INT_TO_FIXED(a), INT_TO_FIXE=
D(b)))
> > -
> >   static size_t pixel_offset(const struct vkms_frame_info *frame_info, =
int x, int y)
> >   {
> >   	return frame_info->offset + (y * frame_info->pitch)
> > @@ -137,19 +122,19 @@ static void RGB565_to_argb_u16(struct line_buffer=
 *stage_buffer,
> >   	int x_limit =3D min_t(size_t, drm_rect_width(&frame_info->dst),
> >   			       stage_buffer->n_pixels);
> > -	s32 fp_rb_ratio =3D INT_TO_FIXED_DIV(65535, 31);
> > -	s32 fp_g_ratio =3D INT_TO_FIXED_DIV(65535, 63);
> > +	s32 fp_rb_ratio =3D drm_fixp_div(drm_int2fixp(65535), 31);
> > +	s32 fp_g_ratio =3D drm_fixp_div(drm_int2fixp(65535), 63);
>=20
> I think you need to add `drm_int2fixp` to 31 and 63.
>=20
> >   	for (size_t x =3D 0; x < x_limit; x++, src_pixels++) {
> >   		u16 rgb_565 =3D le16_to_cpu(*src_pixels);
> > -		s32 fp_r =3D INT_TO_FIXED((rgb_565 >> 11) & 0x1f);
> > -		s32 fp_g =3D INT_TO_FIXED((rgb_565 >> 5) & 0x3f);
> > -		s32 fp_b =3D INT_TO_FIXED(rgb_565 & 0x1f);
> > +		s32 fp_r =3D drm_int2fixp((rgb_565 >> 11) & 0x1f);
> > +		s32 fp_g =3D drm_int2fixp((rgb_565 >> 5) & 0x3f);
> > +		s32 fp_b =3D drm_int2fixp(rgb_565 & 0x1f);
>=20
> And we are cast implicitly from 64 bits int to 32 bits which is
> implementation-defined AFAIK. So, probably we should be using `s64` for a=
ll
> of these variables.
>=20
> I tested the patch. And I'm seeing some differences in the intermediate
> results. From my testing, these changes solve those differences.

Hi Igor,

Thanks for checking the calc results and all inputs provided.  I just
sent a second version, can you take a look? I replicated your
suggestions for RGB565_to_argb_u16() in argb_u16_to_RGB565() and
double-checked for i386 and arm. Let me know what you think.

>=20
> Another thing that may have an impact on the final output is the lack of
> rounding in drm_fixed.h. This can potentially produce the wrong result.

Yeah, I see... I can include a comment about the rounding issue for
further improvements, or do you plan to work on it?

Thanks,

Melissa
>=20
> Thanks,
> ---
> Igor Torrente
>=20
> >   		out_pixels[x].a =3D (u16)0xffff;
> > -		out_pixels[x].r =3D FIXED_TO_INT_ROUND(FIXED_MUL(fp_r, fp_rb_ratio));
> > -		out_pixels[x].g =3D FIXED_TO_INT_ROUND(FIXED_MUL(fp_g, fp_g_ratio));
> > -		out_pixels[x].b =3D FIXED_TO_INT_ROUND(FIXED_MUL(fp_b, fp_rb_ratio));
> > +		out_pixels[x].r =3D drm_fixp2int(drm_fixp_mul(fp_r, fp_rb_ratio));
> > +		out_pixels[x].g =3D drm_fixp2int(drm_fixp_mul(fp_g, fp_g_ratio));
> > +		out_pixels[x].b =3D drm_fixp2int(drm_fixp_mul(fp_b, fp_rb_ratio));
> >   	}
> >   }
> > @@ -248,17 +233,17 @@ static void argb_u16_to_RGB565(struct vkms_frame_=
info *frame_info,
> >   	int x_limit =3D min_t(size_t, drm_rect_width(&frame_info->dst),
> >   			    src_buffer->n_pixels);
> > -	s32 fp_rb_ratio =3D INT_TO_FIXED_DIV(65535, 31);
> > -	s32 fp_g_ratio =3D INT_TO_FIXED_DIV(65535, 63);
> > +	s32 fp_rb_ratio =3D drm_fixp_div(drm_int2fixp(65535), 31);
> > +	s32 fp_g_ratio =3D drm_fixp_div(drm_int2fixp(65535), 63);
> >   	for (size_t x =3D 0; x < x_limit; x++, dst_pixels++) {
> > -		s32 fp_r =3D INT_TO_FIXED(in_pixels[x].r);
> > -		s32 fp_g =3D INT_TO_FIXED(in_pixels[x].g);
> > -		s32 fp_b =3D INT_TO_FIXED(in_pixels[x].b);
> > +		s32 fp_r =3D drm_int2fixp(in_pixels[x].r);
> > +		s32 fp_g =3D drm_int2fixp(in_pixels[x].g);
> > +		s32 fp_b =3D drm_int2fixp(in_pixels[x].b);
> > -		u16 r =3D FIXED_TO_INT_ROUND(FIXED_DIV(fp_r, fp_rb_ratio));
> > -		u16 g =3D FIXED_TO_INT_ROUND(FIXED_DIV(fp_g, fp_g_ratio));
> > -		u16 b =3D FIXED_TO_INT_ROUND(FIXED_DIV(fp_b, fp_rb_ratio));
> > +		u16 r =3D drm_fixp2int(drm_fixp_div(fp_r, fp_rb_ratio));
> > +		u16 g =3D drm_fixp2int(drm_fixp_div(fp_g, fp_g_ratio));
> > +		u16 b =3D drm_fixp2int(drm_fixp_div(fp_b, fp_rb_ratio));
> >   		*dst_pixels =3D cpu_to_le16(r << 11 | g << 5 | b);
> >   	}
>=20

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