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

Geert Uytterhoeven
2021-07-20T07:57:47Z

Hi Sam,

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.

> > @@ -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...