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

[Geert Uytterhoeven]

Hi Michael,

On Mon, 7 Sept 2026 at 22:14, Michael Schmitz <schmitzmic@xxxxxxxxx> wrote:
> On 7/09/26 23:14, Miro Kropáček wrote:
> > On Sun, 6 Sept 2026 at 11:49, Michael Schmitz <schmitzmic@xxxxxxxxx> wrote:
> >> Hi Linmao,
> >>
> >> thanks for your patch!
> >>
> >> 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);
> >> +               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.
> > Linmao's patch: I ran it on a Falcon with SuperVidel at
> > 1920x1080x32bpp, all good.
> >
> > Michael's 24bpp fill: I cannot test it because the SuperVidel has no
> > 24bpp video mode. It was planned but never implemented. Its mode
> > register offers 1/2/4/8-bit bitplane, 8-bit chunky, 16-bit highcolor
> > and 32-bit truecolor (24-bit RGB plus an alpha byte), with the
> > remaining code reserved - and TOS offers no 24bpp mode to set.
> Without a 24 bpp mode, the 24 bpp fill is pointless, so no point trying
> to keep that mode alive.
> > Unless I overlooked something, the fill also looks wrong on paper: the
> > first line is filled with a repeated byte and then with repeated
> > 32-bit words of the same value, which cannot reproduce a 3-byte pixel
> > pattern unless all three bytes are equal.
>
> You're correct - we'd need a memset24() to account for that. Without

Unless the frame buffer would be using a DIRECT_COLOR visual (which
is not the case), then you could draw pixels in the 16 console colors
by replicating the color index in all 3 bytes ;-)

> hardware support this is an academic exercise that I don't have time for :-)

True. And as this code is used only to fill the first line (the blitter
replicates it into the other lines), the performance difference between
a simple byte loop and a true memset24() won't be that large.


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