linux-kernel - Re: [PATCH resend 2/5] video: fbdev: ssd1307fb: Simplify ssd1307fb_update

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Date:   Tue, 20 Jul 2021 09:43:22 +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 2/5] video: fbdev: ssd1307fb: Simplify ssd1307fb_update_display()

Hi Sam,

On Mon, Jul 19, 2021 at 9:05 PM Sam Ravnborg <sam@...nborg.org> wrote:
> On Wed, Jul 14, 2021 at 04:58:01PM +0200, Geert Uytterhoeven wrote:
> > Simplify the nested loops to handle conversion from linear frame buffer
> > to ssd1307 page layout:
> >   1. Move last page handling one level up, as the value of "m" is the
> >      same inside a page,
> >   2. array->data[] is filled linearly, so there is no need to
> >      recalculate array_idx over and over again; a simple increment is
> >      sufficient.
> >
> > Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> > ---
> >  drivers/video/fbdev/ssd1307fb.c | 17 ++++++++++-------
> >  1 file changed, 10 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/video/fbdev/ssd1307fb.c b/drivers/video/fbdev/ssd1307fb.c
> > index e6b6263e3bef847f..6d7bd025bca1a175 100644
> > --- a/drivers/video/fbdev/ssd1307fb.c
> > +++ b/drivers/video/fbdev/ssd1307fb.c
> > @@ -158,6 +158,7 @@ static int ssd1307fb_update_display(struct ssd1307fb_par *par)
> >       u8 *vmem = par->info->screen_buffer;
> >       unsigned int line_length = par->info->fix.line_length;
> >       unsigned int pages = DIV_ROUND_UP(par->height, 8);
> > +     u32 array_idx = 0;
> >       int ret, i, j, k;
> >
> >       array = ssd1307fb_alloc_array(par->width * pages, SSD1307FB_DATA);
> > @@ -194,19 +195,21 @@ static int ssd1307fb_update_display(struct ssd1307fb_par *par)
> >        */
> >
> >       for (i = 0; i < pages; i++) {
> > +             int m = 8;
> > +
> > +             /* Last page may be partial */
> > +             if (i + 1 == pages && par->height % 8)
> > +                     m = par->height % 8;
> >               for (j = 0; j < par->width; j++) {
> > -                     int m = 8;
> > -                     u32 array_idx = i * par->width + j;
> > -                     array->data[array_idx] = 0;
> > -                     /* Last page may be partial */
> > -                     if (i + 1 == pages && par->height % 8)
> > -                             m = par->height % 8;
> > +                     u8 data = 0;
> > +
> >                       for (k = 0; k < m; k++) {
> If the last page is partial then m will be less than 8 for all bytes in
> j = 0..par-width - but m should only be less than 8 for the last
> iteration of the loop.
>
> Do I miss something or is the code buggy?

"the loop" is the j-loop?
If m is less than 8 for the last page, it should be less than 8 for
all iterations of j, as all last bytes in each "line" (visible
row) are partial, cfr. the comments above the code, explaining the
representation of the screen.

> >                               u8 byte = vmem[(8 * i + k) * line_length +
> >                                              j / 8];
> >                               u8 bit = (byte >> (j % 8)) & 1;
> > -                             array->data[array_idx] |= bit << k;
> > +                             data |= bit << k;
> >                       }
> > +                     array->data[array_idx++] = data;
> >               }
> >       }

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