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

[Javier Martinez Canillas <javierm@redhat.com>]

Hello Thomas,

On 2/17/22 11:34, 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.
>

Wow, that's a big speedup!

> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> ---
>  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;
> +

Also the code is much more simpler / easy to read now. Amazing patch.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>

Best regards,
-- 
Javier Martinez Canillas
Linux Engineering
Red Hat