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

Hi Sam,

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

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

--- 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.

quoted

@@ -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@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