[igt-dev] [PATCH] tests/kms_writeback.c: fix the `fill_fb` function

[Petri Latvala]

On Tue, Oct 05, 2021 at 10:56:01AM -0300, Igor Matheus Andrade Torrente wrote:
> Currently, the `fill_fb` function, instead of filling the framebuffer
> with the whole pixel, is only filling it with the rightmost byte.
> 
> This is happening because, even though the memset accepts an int as
> a parameter, it's casting the value to unsigned char and will fill
> the memory with this 8 bits value.
> 
> If we setup the pixel to 0xff0000|f0|, the buffer will filled with:
> 
> | addr     | = 0f
> | addr + 1 | = 0f
> | addr + 2 | = 0f
> | addr + 3 | = 0f
>     ...
> | addr + N | = 0f
> 
> This patch address this issue by manually copying the entire integer
> in the little-endian format.
> 
> Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente at gmail.com>
> ---
>  tests/kms_writeback.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/tests/kms_writeback.c b/tests/kms_writeback.c
> index 60dbda2a..60d9b74e 100644
> --- a/tests/kms_writeback.c
> +++ b/tests/kms_writeback.c
> @@ -227,14 +227,26 @@ static void writeback_fb_id(igt_output_t *output, igt_fb_t *valid_fb, igt_fb_t *
>  
>  static void fill_fb(igt_fb_t *fb, uint32_t pixel)
>  {
> -	void *ptr;
> +	uint8_t *ptr, x, r, g, b, step = sizeof(uint32_t);
> +	int64_t pixel_count, i;
>  
>  	igt_assert(fb->drm_format == DRM_FORMAT_XRGB8888);
>  
>  	ptr = igt_fb_map_buffer(fb->fd, fb);
>  	igt_assert(ptr);
>  
> -	memset(ptr, pixel, fb->strides[0] * fb->height);
> +	pixel_count = fb->strides[0] * fb->height;
> +	x = (pixel & (0xff << 24)) >> 24;
> +	r = (pixel & (0xff << 16)) >> 16;
> +	g = (pixel & (0xff << 8)) >> 8;
> +	b = pixel & 0xff;
> +
> +	for (i = 0; i <= pixel_count - step; i += step) {
> +		ptr[i + 3] = x;
> +		ptr[i + 2] = r;
> +		ptr[i + 1] = g;
> +		ptr[i + 0] = b;
> +	}

Is there a reason to explicitly go for little endian here? Can't the
pixel be written as is, like

uint32_t *p = ptr;

while (p++ < ptr + fb->strides[0] * fb->height)
  *p = pixel;


-- 
Petri Latvala