linux-kernel - Re: [PATCH 2/5] drm/format-helper: Fix XRGB888 to monochrome conversion

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Date:   Tue, 15 Mar 2022 13:48:48 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Javier Martinez Canillas <javierm@...hat.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Noralf Trønnes <noralf@...nnes.org>,
        Pekka Paalanen <pekka.paalanen@...labora.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] drm/format-helper: Fix XRGB888 to monochrome conversion

Hi Javier,

On Tue, Mar 15, 2022 at 1:18 PM Javier Martinez Canillas
<javierm@...hat.com> wrote:
> On 3/15/22 12:07, Geert Uytterhoeven wrote:
> > The conversion functions drm_fb_xrgb8888_to_mono() and
> > drm_fb_gray8_to_mono_line() do not behave correctly when the
> > horizontal boundaries of the clip rectangle are not multiples of 8:
> >   a. When x1 % 8 != 0, the calculated pitch is not correct,
> >   b. When x2 % 8 != 0, the pixel data for the last byte is wrong.
> >
>
> Thanks a lot for tracking down and fixing these issues.
>
> > Simplify the code and fix (a) by:
> >   1. Removing start_offset, and always storing the first pixel in the
> >      first bit of the monochrome destination buffer.
> >      Drivers that require the first pixel in a byte to be located at an
> >      x-coordinate that is a multiple of 8 can always align the clip
> >      rectangle before calling drm_fb_xrgb8888_to_mono().
> >      Note that:
> >        - The ssd130x driver does not need the alignment, as the
> >        monochrome buffer is a temporary format,
> >        - The repaper driver always updates the full screen, so the clip
> >        rectangle is always aligned.
> >   2. Passing the number of pixels to drm_fb_gray8_to_mono_line(),
> >      instead of the number of bytes, and the number of pixels in the
> >      last byte.
> >
> > Fix (b) by explicitly setting the target bit, instead of always setting
> > bit 7 and shifting the value in each loop iteration.
> >
> > Remove the bogus pitch check, which operates on bytes instead of pixels,
> > and triggers when e.g. flashing the cursor on a text console with a font
> > that is 8 pixels wide.
> >
> > Drop the confusing comment about scanlines, as a pitch in bytes always
> > contains a multiple of 8 pixels.
> >
> > While at it, use the drm_rect_height() helper instead of open-coding the
> > same operation.
> >
> > Update the comments accordingly.
> >
> > Fixes: bcf8b616deb87941 ("drm/format-helper: Add drm_fb_xrgb8888_to_mono_reversed()")
> > Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> > ---
>
> Acked-by: Javier Martinez Canillas <javierm@...hat.com>

Thanks!

> I just have a small comment below.
>
> [snip]
>
> > +static void drm_fb_gray8_to_mono_line(u8 *dst, const u8 *src, unsigned int pixels)
> > +{
> > +     while (pixels) {
> > +             unsigned int i, bits = min(pixels, 8U);
> > +             u8 byte = 0;
> >
> > -                     byte >>= 1;
> > -                     if (src[x] >> 7)
> > -                             byte |= BIT(7);
> > +             for (i = 0; i < bits; i++, pixels--) {
>
> I think is worth to add a comment here explaining that the pixel is set to
> 1 for brightness > 127 and to 0 for brightness < 128. Or as kernel-doc for
> this helper function.
>
> > +                     if (*src++ & BIT(7))
>
> Pekka also mentioned that if (*src++ > 127) would make this easier to read.

Sure, will update. Nicely removes the need for a comment.

> > +                             byte |= BIT(i);
> >               }
> >               *dst++ = byte;
> >       }

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