Re: [v3,4/5] fbdev: Improve performance of cfb_imageblit()

[Thomas Zimmermann <tzimmermann@xxxxxxx>]

Hi

Am 09.03.22 um 11:39 schrieb Geert Uytterhoeven:
> Hi Marek,
>
> On Wed, Mar 9, 2022 at 10:22 AM Marek Szyprowski
> <m.szyprowski@xxxxxxxxxxx> wrote:
> > On 09.03.2022 09:22, Thomas Zimmermann wrote:
> > > Am 08.03.22 um 23:52 schrieb Marek Szyprowski:
> > > > On 23.02.2022 20:38, Thomas Zimmermann wrote:
> > > > > Improve the performance of cfb_imageblit() by manually unrolling
> > > > > the inner blitting loop and moving some invariants out. The compiler
> > > > > failed to do this automatically. This change keeps cfb_imageblit()
> > > > > in sync with sys_imagebit().
> > > > >
> > > > > A microbenchmark measures the average number of CPU cycles
> > > > > for cfb_imageblit() after a stabilizing period of a few minutes
> > > > > (i7-4790, FullHD, simpledrm, kernel with debugging).
> > > > >
> > > > > cfb_imageblit(), new: 15724 cycles
> > > > > cfb_imageblit(): old: 30566 cycles
> > > > >
> > > > > In the optimized case, cfb_imageblit() is now ~2x faster than before.
> > > > >
> > > > > v3:
> > > > >      * fix commit description (Pekka)
> > > > >
> > > > > Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
> > > > > Acked-by: Sam Ravnborg <sam@xxxxxxxxxxxx>
> > > > > Reviewed-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>
> > > > This patch landed recently in linux next-20220308 as commit 0d03011894d2
> > > > ("fbdev: Improve performance of cfb_imageblit()"). Sadly it causes a
> > > > freeze after DRM and emulated fbdev initialization on various Samsung
> > > > Exynos ARM 32bit based boards. This happens when kernel is compiled from
> > > > exynos_defconfig. Surprisingly when kernel is compiled from
> > > > multi_v7_defconfig all those boards boot fine, so this is a matter of
> > > > one of the debugging options enabled in the exynos_defconfig. I will try
> > > > to analyze this further and share the results. Reverting $subject on top
> > > > of next-20220308 fixes the boot issue.
> > >
> > > Thanks for reporting. I don't have the hardware to reproduce it and
> > > there's no obvious difference to the original version. It's supposed
> > > to be the same algorithm with a different implementation. Unless you
> > > can figure out the issue, we can also revert the patch easily.
> >
> > I've played a bit with .config options and found that the issue is
> > caused by the compiled-in fonts used for the framebuffer. For some
> > reasons (so far unknown to me), exynos_defconfig has the following odd
> > setup:
> >
> > CONFIG_FONT_SUPPORT=y
> > CONFIG_FONTS=y
> > # CONFIG_FONT_8x8 is not set
> > # CONFIG_FONT_8x16 is not set
> > # CONFIG_FONT_6x11 is not set
> > CONFIG_FONT_7x14=y
> > # CONFIG_FONT_PEARL_8x8 is not set
> > # CONFIG_FONT_ACORN_8x8 is not set
> > # CONFIG_FONT_MINI_4x6 is not set
> > # CONFIG_FONT_6x10 is not set
> > # CONFIG_FONT_10x18 is not set
> > # CONFIG_FONT_SUN8x16 is not set
> > # CONFIG_FONT_SUN12x22 is not set
> > # CONFIG_FONT_TER16x32 is not set
> > # CONFIG_FONT_6x8 is not set
> >
> > Such setup causes a freeze during framebuffer initialization (or just
> > after it got registered). I've reproduced this even on Raspberry Pi 3B
> > with multi_v7_defconfig and changed fonts configuration (this also
> > required to disable vivid driver, which forces 8x16 font), where I got
> > the following panic:
> >
> > simple-framebuffer 3eace000.framebuffer: framebuffer at 0x3eace000,
> > 0x12c000 bytes
> > simple-framebuffer 3eace000.framebuffer: format=a8r8g8b8,
> > mode=640x480x32, linelength=2560
> > 8<--- cut here ---
> > Unable to handle kernel paging request at virtual address f0aac000
>
> So support for images with offsets or widths that are not a multiple
> of 8 got broken in cfb_imageblit(). Oops...
>
> BTW, the various drawing routines used to set a bitmask indicating
> which alignments were supported (see blit_x), but most of them no
> longer do, presumably because all alignments are now supported
> (since ca. 20 years?).
> So you can (temporarily) work around this by filling in blit_x,
> preventing the use of the 7x14 font.

How do I activate the 7x14 font? It's compiled into the kernel already
(CONFIG_FONT_7x14=y).

Best regards
Thomas

> Gr{oetje,eeting}s,
>
>                          Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                  -- Linus Torvalds

--
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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