linux-kernel - Re: [PATCH resend 4/5] video: fbdev: ssd1307fb: Optimize screen updates

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Date:   Tue, 20 Jul 2021 09:56:52 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Sam Ravnborg <sam@...nborg.org>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        Maxime Ripard <mripard@...nel.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH resend 4/5] video: fbdev: ssd1307fb: Optimize screen updates

Hi Sam,

On Mon, Jul 19, 2021 at 9:21 PM Sam Ravnborg <sam@...nborg.org> wrote:
> On Wed, Jul 14, 2021 at 04:58:03PM +0200, Geert Uytterhoeven wrote:
> > Currently, each screen update triggers an I2C transfer of all screen
> > data, up to 1 KiB of data for a 128x64 display, which takes at least 20
> > ms in Fast mode.
> >
> > Reduce the amount of transferred data by only updating the rectangle
> > that changed.  Remove the call to ssd1307fb_set_address_range() during
> > initialization, as ssd1307fb_update_rect() now takes care of that.
> >
> > Note that for now the optimized operation is only used for fillrect,
> > copyarea, and imageblit, which are used by fbcon.
> >
> > Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>

> > --- a/drivers/video/fbdev/ssd1307fb.c
> > +++ b/drivers/video/fbdev/ssd1307fb.c
> > @@ -184,16 +184,18 @@ static int ssd1307fb_set_address_range(struct ssd1307fb_par *par, u8 col_start,
> >       return ssd1307fb_write_cmd(par->client, page_end);
> >  }
> >
> > -static int ssd1307fb_update_display(struct ssd1307fb_par *par)
> > +static int ssd1307fb_update_rect(struct ssd1307fb_par *par, unsigned int x,
> > +                              unsigned int y, unsigned int width,
> > +                              unsigned int height)
> >  {
> >       struct ssd1307fb_array *array;
> >       u8 *vmem = par->info->screen_buffer;
> >       unsigned int line_length = par->info->fix.line_length;
> > -     unsigned int pages = DIV_ROUND_UP(par->height, 8);
> > +     unsigned int pages = DIV_ROUND_UP(height + y % 8, 8);
>
> Add () like this - at least it helps me:
> > +     unsigned int pages = DIV_ROUND_UP((height + y) % 8, 8);

Thanks, that's actually a genuine bug.

> > @@ -226,13 +228,18 @@ static int ssd1307fb_update_display(struct ssd1307fb_par *par)
> >        *  (5) A4 B4 C4 D4 E4 F4 G4 H4
> >        */
> >
> > -     for (i = 0; i < pages; i++) {
> > +     ret = ssd1307fb_set_address_range(par, par->col_offset + x, width,
> > +                                       par->page_offset + y / 8, pages);
> > +     if (ret < 0)
> > +             goto out_free;
> > +
> > +     for (i = y / 8; i < y / 8 + pages; i++) {
> >               int m = 8;
> >
> >               /* Last page may be partial */
> > -             if (i + 1 == pages && par->height % 8)
> > +             if (8 * (i + 1) > par->height)
> >                       m = par->height % 8;
> As before, this looks wrong to me.

Let's sort that out in the other thread...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

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