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

[Michael Schmitz <schmitzmic@gmail.com>]

Hi Geert,

On 7/09/26 20:38, Geert Uytterhoeven wrote:
> Hi Michael,
>
> CC linux-m68k
>
> On Sun, 6 Sept 2026 at 03:49, Michael Schmitz <schmitzmic@gmail.com> 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.

True enough (iff line happens to be aligned, which is only true if 
rect->dx is even).

But as Miro pointed out, 24 bpp can't actually be used (and without TOS 
setting such a mode before Linux boot, it can't ever happen).

Cheers,

     Michael


>
>> +               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@kylinos.cn>
>>> ---
>>>   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
>