[Intel-gfx] DG1 VRAM question

[Dave Airlie]

On Tue, 7 Jul 2020 at 18:36, Matthew Auld <matthew.auld at intel.com> wrote:
>
> On 01/07/2020 00:27, Dave Airlie wrote:
> > On Sat, 27 Jun 2020 at 03:17, Daniele Ceraolo Spurio
> > <daniele.ceraolospurio at intel.com> wrote:
> >>
> >>
> >>
> >> On 6/26/20 12:14 AM, Lucas De Marchi wrote:
> >>> Cc Matt and Daniele
> >>>
> >>> On Thu, Jun 25, 2020 at 9:38 PM Dave Airlie <airlied at gmail.com> wrote:
> >>>>
> >>>> I can't figure this out easily so I'd thought I'd just ask, but does
> >>>> DG1 have VRAM > PCIE aperture, I'm not sure I've see any mention of
> >>>
> >>> We'd need to go via lmem since there's no mappable aperture. There are
> >>> a few patches in tree for that
> >>> (see e.g. 54b512cd7a6d ("drm/i915: do not map aperture if it is not
> >>> available.")) but more missing.
> >>>
> >>
> >> To clarify, although the legacy aperture mapping that allowed the CPU to
> >> access memory via the GGTT for swizzling is gone, VRAM/LMEM is still
> >> cpu-mappable via pci bar.
> >> Will leave the questions about possible trashing to Matt as he's more
> >> familiar than me with how this works.
> >
> > Matt?
> >
> > Is DG1 assuming we can get 64-bit BARs always and the CPU will have
> > access to the complete VRAM? or is there any ideas about what happens
> > in those situations where 64-bit BARs aren't available and there is
> > memory pressure on the PCI BAR space.
> >
> > With other discrete GPUs we've got lots of things like visible VRAM
> > limitations, writing page tables with GPU hw instead of from the CPU,
> > having mapping bring things into the visible area, so you can stream
> > something into VRAM, but then it'll migrated to non-visible area if
> > it's unmapped and there is memory pressure.
>
> Yes, we just assume that LMEM size == LMEMBAR size, where the whole
> thing is 1:1 mapped and CPU visible. We don't currently have the concept
> of CPU visible/non-visible LMEM.

Okay, so this GPU is happy never being supported when connected to
anything where the firmware doesn't enable 64-bit BARs?

I'd love to say 64-bit BARs were everywhere, I suppose in some worlds
BIOSes and OS might even work flawlessly with them enabled. Considered
plugging one of these in over thunderbolt? (maybe TB always gets a
64-bit BAR).

Dave.