Re: [PATCH 1/2] fbdev: Improve performance of sys_fillrect()

[Sam Ravnborg <sam@ravnborg.org>]

Hi Thomas,

On Thu, Feb 17, 2022 at 11:34:04AM +0100, Thomas Zimmermann wrote:
> Improve the performance of sys_fillrect() by using word-aligned
> 32/64-bit mov instructions. While the code tried to implement this,
> the compiler failed to create fast instructions. The resulting
> binary instructions were even slower than cfb_fillrect(), which
> uses the same algorithm, but operates on I/O memory.
> 
> A microbenchmark measures the average number of CPU cycles
> for sys_fillrect() 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_fillrect(), new:  26586 cycles
>   sys_fillrect(), old: 166603 cycles
>   cfb_fillrect():       41012 cycles
> 
> In the optimized case, sys_fillrect() is now ~6x faster than before
> and ~1.5x faster than the CFB implementation.
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>

Nice optimization.
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>

> ---
>  drivers/video/fbdev/core/sysfillrect.c | 16 +++-------------
>  1 file changed, 3 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/video/fbdev/core/sysfillrect.c b/drivers/video/fbdev/core/sysfillrect.c
> index 33ee3d34f9d2..bcdcaeae6538 100644
> --- a/drivers/video/fbdev/core/sysfillrect.c
> +++ b/drivers/video/fbdev/core/sysfillrect.c
> @@ -50,19 +50,9 @@ bitfill_aligned(struct fb_info *p, unsigned long *dst, int dst_idx,
>  
>  		/* Main chunk */
>  		n /= bits;
> -		while (n >= 8) {
> -			*dst++ = pat;
> -			*dst++ = pat;
> -			*dst++ = pat;
> -			*dst++ = pat;
> -			*dst++ = pat;
> -			*dst++ = pat;
> -			*dst++ = pat;
> -			*dst++ = pat;
> -			n -= 8;
> -		}
> -		while (n--)
> -			*dst++ = pat;
> +		memset_l(dst, pat, n);
> +		dst += n;
> +
>  		/* Trailing bits */
>  		if (last)
>  			*dst = comp(pat, *dst, last);
> -- 
> 2.34.1