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

Hi Sam,

On Tue, Jul 20, 2021 at 9:56 AM Geert Uytterhoeven [off-list ref] wrote:

On Mon, Jul 19, 2021 at 9:21 PM Sam Ravnborg [off-list ref] wrote:

quoted

On Wed, Jul 14, 2021 at 04:58:03PM +0200, Geert Uytterhoeven wrote:

quoted

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>

quoted

quoted

--- 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:

quoted

+     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);

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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