Re: [PATCH] ati-vga: increment mm_index in ati_mm_read/write

[BALATON Zoltan <balaton@eik.bme.hu>]

On Wed, 3 Jun 2020, P J P wrote:
> +-- On Wed, 3 Jun 2020, Gerd Hoffmann wrote --+
> | Hmm, why modify mm_index?  Shouldn't we just check it is non-zero
> | before calling ati_mm_read/ati_mm_write?
>
>  if (s->regs.mm_index & BIT(31)) {
>     ...
>  } else {
>     ati_mm_write(s, s->regs.mm_index + addr - MM_DATA, data, size);
>  }
>
> Exit condition for recursion is to set (mm_index & BIT(31)), so recursion
> would continue even with non-zero values I think.

MM_INDEX and MM_DATA allows access to registers (or memory if BIT(31) is 
set) via only these two registers. This is called indexed access in docs. 
So it does not really make sense to allow access to these registers as 
well thus we can avoid infinite recursion. It's not meant to recurse until 
BIT(31) is set. I think you can do:

   if (s->regs.mm_index & BIT(31)) {
      ...
-  } else {
+  } else if (s->regs.mm_index >= BIOS_0_SCRATCH) {
      ati_mm_write(s, s->regs.mm_index + addr - MM_DATA, data, size);
   }

and do the same in ati_mm_read() as well.

Regards,
BALATON Zoltan