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

[Sam Ravnborg <sam@ravnborg.org>]

On Tue, Jul 20, 2021 at 11:16:27AM +0200, Geert Uytterhoeven wrote:
> Hi Sam,
> 
> On Tue, Jul 20, 2021 at 9:56 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > On Mon, Jul 19, 2021 at 9:21 PM Sam Ravnborg <sam@ravnborg.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@linux-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.
> 
> No it's not "(height + y) % 8" is wrong.
> 
> Better if I reorder the operands like below?
> 
>     unsigned int pages = DIV_ROUND_UP(y % 8 + height, 8);

Yep, then I would not be fooled so easy.

	Sam