Re: [PATCH] drm/vkms: fix 32bit compilation error by replacing macros

[Melissa Wen]

On 09/09, Igor Matheus Andrade Torrente wrote:
> Hi Mellisa,
> 
> Thanks for the patch fixing my mistakes.
> 
> 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.
> > 
> > Fixes: a19c2ac9858 ("drm: vkms: Add support to the RGB565 format")
> > Tested-by: Sudip Mukherjee <sudipm.mukherjee@xxxxxxxxx>
> > Reported-by: Sudip Mukherjee <sudipm.mukherjee@xxxxxxxxx>
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > Signed-off-by: Melissa Wen <mwen@xxxxxxxxxx>
> > ---
> >   drivers/gpu/drm/vkms/vkms_formats.c | 53 +++++++++++------------------
> >   1 file changed, 19 insertions(+), 34 deletions(-)
> > 
> > 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 fractional
> > - * 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_FIXED(b)))
> > -#define INT_TO_FIXED_DIV(a, b) (FIXED_DIV(INT_TO_FIXED(a), INT_TO_FIXED(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 = min_t(size_t, drm_rect_width(&frame_info->dst),
> >   			       stage_buffer->n_pixels);
> > -	s32 fp_rb_ratio = INT_TO_FIXED_DIV(65535, 31);
> > -	s32 fp_g_ratio = INT_TO_FIXED_DIV(65535, 63);
> > +	s32 fp_rb_ratio = drm_fixp_div(drm_int2fixp(65535), 31);
> > +	s32 fp_g_ratio = drm_fixp_div(drm_int2fixp(65535), 63);
> 
> I think you need to add `drm_int2fixp` to 31 and 63.
> 
> >   	for (size_t x = 0; x < x_limit; x++, src_pixels++) {
> >   		u16 rgb_565 = le16_to_cpu(*src_pixels);
> > -		s32 fp_r = INT_TO_FIXED((rgb_565 >> 11) & 0x1f);
> > -		s32 fp_g = INT_TO_FIXED((rgb_565 >> 5) & 0x3f);
> > -		s32 fp_b = INT_TO_FIXED(rgb_565 & 0x1f);
> > +		s32 fp_r = drm_int2fixp((rgb_565 >> 11) & 0x1f);
> > +		s32 fp_g = drm_int2fixp((rgb_565 >> 5) & 0x3f);
> > +		s32 fp_b = drm_int2fixp(rgb_565 & 0x1f);
> 
> 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 all
> of these variables.
> 
> 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.

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

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