Re: [PATCH 2/2] fbdev: Improve performance of sys_imageblit()

[Sam Ravnborg <sam@ravnborg.org>]

Hi Thomas,

On Thu, Feb 17, 2022 at 11:34:05AM +0100, Thomas Zimmermann wrote:
> Improve the performance of sys_imageblit() by manually unrolling
> the inner blitting loop and moving some invariants out. The compiler
> failed to do this automatically. The resulting binary code was even
> slower than the cfb_imageblit() helper, which uses the same algorithm,
> but operates on I/O memory.

It would be super to have the same optimization done to cfb_imageblit(),
to prevent that the two codebases diverge more than necessary.
Also I think cfb_ version would also see a performance gain from this.

The actual implementation looks good.
So with or without the extra un-rolling the patch is:
Acked-by: Sam Ravnborg <sam@ravnborg.org>

One small nit belwo.

	Sam

> 
> A microbenchmark measures the average number of CPU cycles
> for sys_imageblit() after a stabilizing period of a few minutes
> (i7-4790, FullHD, simpledrm, kernel with debugging). The value
> for CFB is given as a reference.
> 
>   sys_imageblit(), new: 25934 cycles
>   sys_imageblit(), old: 35944 cycles
>   cfb_imageblit():      30566 cycles
> 
> In the optimized case, sys_imageblit() is now ~30% faster than before
> and ~20% faster than cfb_imageblit().
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> ---
>  drivers/video/fbdev/core/sysimgblt.c | 51 +++++++++++++++++++++-------
>  1 file changed, 39 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/video/fbdev/core/sysimgblt.c b/drivers/video/fbdev/core/sysimgblt.c
> index a4d05b1b17d7..d70d65af6fcb 100644
> --- a/drivers/video/fbdev/core/sysimgblt.c
> +++ b/drivers/video/fbdev/core/sysimgblt.c
> @@ -188,23 +188,32 @@ static void fast_imageblit(const struct fb_image *image, struct fb_info *p,
>  {
>  	u32 fgx = fgcolor, bgx = bgcolor, bpp = p->var.bits_per_pixel;
>  	u32 ppw = 32/bpp, spitch = (image->width + 7)/8;
> -	u32 bit_mask, end_mask, eorx, shift;
> +	u32 bit_mask, eorx;
>  	const char *s = image->data, *src;
>  	u32 *dst;
> -	const u32 *tab = NULL;
> -	int i, j, k;
> +	const u32 *tab;
> +	size_t tablen;
> +	u32 colortab[16];
> +	int i, j, k, jdecr;
> +
> +	if ((uintptr_t)dst1 % 8)
> +		return;
This check is new - and should not trigger ever. Maybe add an unlikely
and a WARN_ON_ONCE()?


>  
>  	switch (bpp) {
>  	case 8:
>  		tab = fb_be_math(p) ? cfb_tab8_be : cfb_tab8_le;
> +		tablen = 16;
>  		break;
>  	case 16:
>  		tab = fb_be_math(p) ? cfb_tab16_be : cfb_tab16_le;
> +		tablen = 4;
>  		break;
>  	case 32:
> -	default:
>  		tab = cfb_tab32;
> +		tablen = 2;
>  		break;
> +	default:
> +		return;
>  	}
>  
>  	for (i = ppw-1; i--; ) {
> @@ -217,19 +226,37 @@ static void fast_imageblit(const struct fb_image *image, struct fb_info *p,
>  	bit_mask = (1 << ppw) - 1;
>  	eorx = fgx ^ bgx;
>  	k = image->width/ppw;
> +	jdecr = 8 / ppw;
> +
> +	for (i = 0; i < tablen; ++i)
> +		colortab[i] = (tab[i] & eorx) ^ bgx;
This code could have been embedded with the switch (bpp) {
That would have made some sense I think.
But both ways works, so this was just a small observation.

	Sam