Re: [Qemu-devel] [PATCH] ati-vga: Add registers for getting apertures

[Gerd Hoffmann]

On Fri, Jul 05, 2019 at 12:02:26PM +0200, BALATON Zoltan wrote:
> Some drivers (e.g. Linux radeon drm and MacOS) access these to find
> apertures to access card. Try to implement these but not sure these
> are correct yet.

> +    case CONFIG_APER_SIZE:
> +        val = s->vga.vram_size;
> +        break;

Given the math linux is doing I don't think this is correct:

static u32 r100_get_accessible_vram(struct radeon_device *rdev)
{
        u32 aper_size;
        u8 byte;

        aper_size = RREG32(RADEON_CONFIG_APER_SIZE);

        [ ... ]

        /* Single function older card. We read HDP_APER_CNTL to see how the BIOS
         * have set it up. We don't write this as it's broken on some ASICs but
         * we expect the BIOS to have done the right thing (might be too 
optimistic...)
         */
        if (RREG32(RADEON_HOST_PATH_CNTL) & RADEON_HDP_APER_CNTL)
                return aper_size * 2;
        return aper_size;
}

IIRC old ati cards had a pci bar twice the size of the vram, where the lower
half of the pci bar was straight access to the video memory and the upper
half byteswapped access.

I think that is the background of the "aper_size * 2" logic above, and
RADEON_HDP_APER_CNTL is probably a config bit for the behavior.

cheers,
  Gerd