[PATCH 2/5] drm/format-helper: Fix XRGB888 to monochrome conversion

[Javier Martinez Canillas]

On 3/15/22 12:07, Geert Uytterhoeven wrote:
> The conversion functions drm_fb_xrgb8888_to_mono() and
> drm_fb_gray8_to_mono_line() do not behave correctly when the
> horizontal boundaries of the clip rectangle are not multiples of 8:
>   a. When x1 % 8 != 0, the calculated pitch is not correct,
>   b. When x2 % 8 != 0, the pixel data for the last byte is wrong.
>

Thanks a lot for tracking down and fixing these issues.

> Simplify the code and fix (a) by:
>   1. Removing start_offset, and always storing the first pixel in the
>      first bit of the monochrome destination buffer.
>      Drivers that require the first pixel in a byte to be located at an
>      x-coordinate that is a multiple of 8 can always align the clip
>      rectangle before calling drm_fb_xrgb8888_to_mono().
>      Note that:
>        - The ssd130x driver does not need the alignment, as the
> 	 monochrome buffer is a temporary format,
>        - The repaper driver always updates the full screen, so the clip
> 	 rectangle is always aligned.
>   2. Passing the number of pixels to drm_fb_gray8_to_mono_line(),
>      instead of the number of bytes, and the number of pixels in the
>      last byte.
> 
> Fix (b) by explicitly setting the target bit, instead of always setting
> bit 7 and shifting the value in each loop iteration.
> 
> Remove the bogus pitch check, which operates on bytes instead of pixels,
> and triggers when e.g. flashing the cursor on a text console with a font
> that is 8 pixels wide.
> 
> Drop the confusing comment about scanlines, as a pitch in bytes always
> contains a multiple of 8 pixels.
> 
> While at it, use the drm_rect_height() helper instead of open-coding the
> same operation.
> 
> Update the comments accordingly.
> 
> Fixes: bcf8b616deb87941 ("drm/format-helper: Add drm_fb_xrgb8888_to_mono_reversed()")
> Signed-off-by: Geert Uytterhoeven <geert at linux-m68k.org>
> ---

Acked-by: Javier Martinez Canillas <javierm at redhat.com>

I just have a small comment below.

[snip]

> +static void drm_fb_gray8_to_mono_line(u8 *dst, const u8 *src, unsigned int pixels)
> +{
> +	while (pixels) {
> +		unsigned int i, bits = min(pixels, 8U);
> +		u8 byte = 0;
>  
> -			byte >>= 1;
> -			if (src[x] >> 7)
> -				byte |= BIT(7);
> +		for (i = 0; i < bits; i++, pixels--) {

I think is worth to add a comment here explaining that the pixel is set to
1 for brightness > 127 and to 0 for brightness < 128. Or as kernel-doc for
this helper function.

> +			if (*src++ & BIT(7))

Pekka also mentioned that if (*src++ > 127) would make this easier to read.

> +				byte |= BIT(i);
>  		}
>  		*dst++ = byte;
>  	}

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat