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

[Thomas Zimmermann]

Hi Sam

Am 18.02.22 um 11:14 schrieb Sam Ravnborg:

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.

Yes, I can do that.

The actual implementation looks good.
So with or without the extra un-rolling the patch is:
Acked-by: Sam Ravnborg <s...@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 <tzimmerm...@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()?

Best regards
Thomas

I think I can remove this test. It was supposed to tell the compiler that dst1 is 8-aligned, but I don't think it worked.

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;
        }

@@ -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;

for (i = ppw-1; i--; ) {

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

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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