Re: [PATCH 8/8] drm/i915: Determine the stolen memory base address on gen2

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Nov 28, 2013 at 04:32:14PM +0000, Chris Wilson wrote:
> On Thu, Nov 28, 2013 at 05:15:10PM +0200, ville.syrjala@linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > There doesn't seem to an explicit stolen memory base register on gen2.
> > Some old comment in the code suggests we should get it via
> > max_low_pfn_mapped, but that's clearly a bad idea on my MGM.
> > 
> > The e820 map in said machine looks like this:
> > [    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009f7ff] usable
> > [    0.000000] BIOS-e820: [mem 0x000000000009f800-0x000000000009ffff] reserved
> > [    0.000000] BIOS-e820: [mem 0x00000000000ce000-0x00000000000cffff] reserved
> > [    0.000000] BIOS-e820: [mem 0x00000000000dc000-0x00000000000fffff] reserved
> > [    0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000001f6effff] usable
> > [    0.000000] BIOS-e820: [mem 0x000000001f6f0000-0x000000001f6f7fff] ACPI data
> > [    0.000000] BIOS-e820: [mem 0x000000001f6f8000-0x000000001f6fffff] ACPI NVS
> > [    0.000000] BIOS-e820: [mem 0x000000001f700000-0x000000001fffffff] reserved
> > [    0.000000] BIOS-e820: [mem 0x00000000fec10000-0x00000000fec1ffff] reserved
> > [    0.000000] BIOS-e820: [mem 0x00000000ffb00000-0x00000000ffbfffff] reserved
> > [    0.000000] BIOS-e820: [mem 0x00000000fff00000-0x00000000ffffffff] reserved
> > 
> > That makes max_low_pfn_mapped = 1f6f0000, so assuming our stolen memory
> > would start there would place it on top of some ACPI memory regions.
> > So not a good idea as already stated.
> > 
> > The 9MB region after the ACPI regions at 0x1f700000 however looks
> > promising given that the macine reports the stolen memory size to be
> > 8MB. Looking at the PGTBL_CTL register, the GTT entries are at offset
> > 0x1fee00000, and given that the GTT entries occupy 128KB, it looks like
> > the stolen memory could start at 0x1f700000 and the GTT entries would
> > occupy the last 128KB of the stolen memory. I have no idea about the
> > extra 1MB after the GTT entries.
> 
> The base address for stolen memory is explicitly Top of Memory on gen2,
> at least in 830/845 cspecs that I found (and recall). But this idea is
> interesting...

I have a hard time beliveing people would be nuts enough to place
the GTT entries somewhere the middle of the stolen memory area rather
than at the end. But I suppose I could be wrong.

There's also one additional reason why I suspect the last 1MB (or .5MB
on Daniel's machine) must be something else; it's covered by a UC MTRR,
whereas the presumed stolen memory range is not.

-- 
Ville Syrjälä
Intel OTC