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

[Michael Schmitz <schmitzmic@gmail.com>]

Hi Miro,

On 7/09/26 23:14, Miro Kropáček wrote:
> Hi Linmao, hi Michael,
>
> On Sun, 6 Sept 2026 at 11:49, Michael Schmitz <schmitzmic@gmail.com> 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 
hardware support this is an academic exercise that I don't have time for :-)

> On the fbset question: no, the pixel format, depth and geometry of an
> external framebuffer cannot be changed at runtime here. fbset -depth
> 16 and fbset -xres 1024 -yres 768 both return success and change
> nothing, because ext_decode_var() only validates against the values
> from the boot argument, ext_encode_var() re-derives every field from
> the external_* globals and ext_set_par() is empty. Only a reboot with
> a different atafb:external: argument changes the mode.

All good then - do you need my Acked-by for Linmao's patch, Helge?

Cheers,

Michael