Re: [2/2] fbdev: Fix cfb_imageblit() for arbitrary image widths

[Guenter Roeck <linux@roeck-us.net>]

On Sun, Mar 13, 2022 at 08:29:52PM +0100, Thomas Zimmermann wrote:
> Commit 0d03011894d2 ("fbdev: Improve performance of cfb_imageblit()")
> broke cfb_imageblit() for image widths that are not aligned to 8-bit
> boundaries. Fix this by handling the trailing pixels on each line
> separately. The performance improvements in the original commit do not
> regress by this change.
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> Fixes: 0d03011894d2 ("fbdev: Improve performance of cfb_imageblit()")
> Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: Javier Martinez Canillas <javierm@redhat.com>
> Cc: Sam Ravnborg <sam@ravnborg.org>
> Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>

Tested-by: Guenter Roeck <linux@roeck-us.net>

> ---
>  drivers/video/fbdev/core/cfbimgblt.c | 28 ++++++++++++++++++++++++----
>  1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/video/fbdev/core/cfbimgblt.c b/drivers/video/fbdev/core/cfbimgblt.c
> index 7361cfabdd85..9ebda4e0dc7a 100644
> --- a/drivers/video/fbdev/core/cfbimgblt.c
> +++ b/drivers/video/fbdev/core/cfbimgblt.c
> @@ -218,7 +218,7 @@ static inline void fast_imageblit(const struct fb_image *image, struct fb_info *
>  {
>  	u32 fgx = fgcolor, bgx = bgcolor, bpp = p->var.bits_per_pixel;
>  	u32 ppw = 32/bpp, spitch = (image->width + 7)/8;
> -	u32 bit_mask, eorx;
> +	u32 bit_mask, eorx, shift;
>  	const char *s = image->data, *src;
>  	u32 __iomem *dst;
>  	const u32 *tab = NULL;
> @@ -259,17 +259,23 @@ static inline void fast_imageblit(const struct fb_image *image, struct fb_info *
>  
>  	for (i = image->height; i--; ) {
>  		dst = (u32 __iomem *)dst1;
> +		shift = 8;
>  		src = s;
>  
> +		/*
> +		 * Manually unroll the per-line copying loop for better
> +		 * performance. This works until we processed the last
> +		 * completely filled source byte (inclusive).
> +		 */
>  		switch (ppw) {
>  		case 4: /* 8 bpp */
> -			for (j = k; j; j -= 2, ++src) {
> +			for (j = k; j >= 2; j -= 2, ++src) {
>  				FB_WRITEL(colortab[(*src >> 4) & bit_mask], dst++);
>  				FB_WRITEL(colortab[(*src >> 0) & bit_mask], dst++);
>  			}
>  			break;
>  		case 2: /* 16 bpp */
> -			for (j = k; j; j -= 4, ++src) {
> +			for (j = k; j >= 4; j -= 4, ++src) {
>  				FB_WRITEL(colortab[(*src >> 6) & bit_mask], dst++);
>  				FB_WRITEL(colortab[(*src >> 4) & bit_mask], dst++);
>  				FB_WRITEL(colortab[(*src >> 2) & bit_mask], dst++);
> @@ -277,7 +283,7 @@ static inline void fast_imageblit(const struct fb_image *image, struct fb_info *
>  			}
>  			break;
>  		case 1: /* 32 bpp */
> -			for (j = k; j; j -= 8, ++src) {
> +			for (j = k; j >= 8; j -= 8, ++src) {
>  				FB_WRITEL(colortab[(*src >> 7) & bit_mask], dst++);
>  				FB_WRITEL(colortab[(*src >> 6) & bit_mask], dst++);
>  				FB_WRITEL(colortab[(*src >> 5) & bit_mask], dst++);
> @@ -290,6 +296,20 @@ static inline void fast_imageblit(const struct fb_image *image, struct fb_info *
>  			break;
>  		}
>  
> +		/*
> +		 * For image widths that are not a multiple of 8, there
> +		 * are trailing pixels left on the current line. Print
> +		 * them as well.
> +		 */
> +		for (; j--; ) {
> +			shift -= ppw;
> +			FB_WRITEL(colortab[(*src >> shift) & bit_mask], dst++);
> +			if (!shift) {
> +				shift = 8;
> +				++src;
> +			}
> +		}
> +
>  		dst1 += p->fix.line_length;
>  		s += spitch;
>  	}