Re: [PATCH] fbdev: atafb: Restrict SuperBlitter to supported formats

[Geert Uytterhoeven]

Hi Michael,

CC linux-m68k

On Sun, 6 Sept 2026 at 03:49, Michael Schmitz <schmitzmic@xxxxxxxxx> wrote:
> Am 27.08.2026 um 21:39 schrieb Linmao Li:
> > The SuperBlitter operations derive an integer byte count per pixel.  The
> > accelerated fill path handles only one-, two- and four-byte pixels.
> > However, the operations are currently installed for every external
> > framebuffer in SuperVidel RAM, including planar 1/2/4/8-bpp and 24-bpp
> > truecolor modes accepted by the external video parser.
> >
> > For 1/2/4-bpp modes, the byte count becomes zero, so accelerated copies do
> > nothing and fills fall through to 32-bit stores.  Planar 8-bpp uses an
> > incompatible memory layout.  For 24-bpp modes, fills also use 32-bit stores
> > despite advancing addresses by three bytes per pixel.  These cases can
> > corrupt the framebuffer beyond the requested rectangle.
>
> I believe 24 bpp mode can be rescued using something like this (entirely
> untested):
>
> --- a/drivers/video/fbdev/atafb.c
> +++ b/drivers/video/fbdev/atafb.c
> @@ -2463,6 +2463,11 @@ static void svblit_fillrect(struct fb_info *info,
>          case 2:
>                  memset16((u16 *)line, pix, rect->width);
>                  break;
> +       case 3:
> +               memset(line, pix, ((rect->width * bytespp) % 4));
> +               line += ((rect->width * bytespp) % 4);
> +               memset32((u32 *)line, pix, (rect->width * bytespp) / 4);

This uses unaligned writes for the largest part.
You can avoid that by doing the memset32() first.

> +               break;
>          default:
>                  memset32((u32 *)line, pix, rect->width);
>                  break;
>
> Can't test this on hardware, and would need Miro to confirm it works as
> intended.
>
> >
> > Enable the SuperBlitter operations only for the layouts they implement:
> > 8-bpp packed pixels and 16/32-bpp truecolor.  Keep the existing software
> > operations for all other external formats.
> >
> > Fixes: d463633d63e6 ("fbdev: atafb: Add support for SuperVidel's SuperBlitter")
> > Signed-off-by: Linmao Li <lilinmao@xxxxxxxxxx>
> > ---
> >  drivers/video/fbdev/atafb.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/video/fbdev/atafb.c b/drivers/video/fbdev/atafb.c
> > index 5bca34c45cef3..c3011b61a94b9 100644
> > --- a/drivers/video/fbdev/atafb.c
> > +++ b/drivers/video/fbdev/atafb.c
> > @@ -3360,7 +3360,11 @@ static int __init atafb_probe(struct platform_device *pdev)
> >               memset (screen_base, 0, external_len);
> >
> >               /* framebuffer in SV RAM: enable the SuperBlitter */
> > -             if (external_addr >= 0xa0000000) {
> > +             if (external_addr >= 0xa0000000 &&
> > +                 ((external_pmode == FB_TYPE_PACKED_PIXELS &&
> > +                   external_depth == 8) ||
> > +                  (external_pmode == -1 &&
> > +                   (external_depth == 16 || external_depth == 32)))) {
> >                       svblit_regs = ioremap(SVBLIT_REGS_PHYS, 0x100);
> >                       if (svblit_regs) {
> >                               svblit_fw = svblit_rd(SVBLIT_VERSION) & 0x1ff;
> >
>
> Otherwise, LGTM.
>
> @Geert: can you remember if pixel format or bit depth of an external
> framebuffer can be changed at runtime using fbset??

I am no Atari expert, but IIRC all external framebuffers rely on
being fully set-up before Linux boots?

Gr{oetje,eeting}s,

                        Geert

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

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