Re: [Intel-gfx] [PATCH] drm/i915/display: Fix phys_base to be relative not absolute

[Andrzej Hajda]

On 28.11.2023 04:47, Paz Zcharya wrote:
> On Mon, Nov 27, 2023 at 8:20 PM Paz Zcharya <pazz@xxxxxxxxxxxx> wrote:
> > On 21.11.2023 13:06, Andrzej Hajda wrote:
> > > On 18.11.2023 00:01, Paz Zcharya wrote:
> > > > On Tue, Nov 14, 2023 at 10:13:59PM -0500, Rodrigo Vivi wrote:
> > > > > On Sun, Nov 05, 2023 at 05:27:03PM +0000, Paz Zcharya wrote:
> > > >
> > > > Hi Rodrigo, thanks for the great comments.
> > > >
> > > > Apologies for using a wrong/confusing terminology. I think 'phys_base'
> > > > is supposed to be the offset in the GEM BO, where base (or
> > > > "Surface Base Address") is supposed to be the GTT offset.
> > >
> > > Since base is taken from PLANE_SURF register it should be resolvable via
> > > GGTT to physical address pointing to actual framebuffer.
> > > I couldn't find anything in the specs.
> >
> > It was quite cryptic. I meant I have not found anything about assumption
> > from commit history that for iGPU there should be 1:1 mapping, this is why
> > there was an assignment "phys_base = base". Possibly the assumption is not
> > valid anymore for MTL(?).
> > Without the assumption we need to check GGTT to determine phys address.
> >
> > > The simplest approach would be then do the same as in case of DGFX:
> > >           gen8_pte_t __iomem *gte = to_gt(i915)->ggtt->gsm;
> > >           gen8_pte_t pte;
> > >
> > >           gte += base / I915_GTT_PAGE_SIZE;
> > >
> > >           pte = ioread64(gte);
> > >           phys_base = pte & I915_GTT_PAGE_MASK;
> > >
> > > Regards
> > > Andrzej
>
> Hey Andrzej,
>
> On a second thought, what do you think about something like
>
> +               gen8_pte_t __iomem *gte = to_gt(i915)->ggtt->gsm;
> +               gen8_pte_t pte;
> +               gte += base / I915_GTT_PAGE_SIZE;
> +               pte = ioread64(gte);
> +               pte = pte & I915_GTT_PAGE_MASK;
> +               phys_base = pte - i915->mm.stolen_region->region.start;
>
> The only difference is the last line.

Bingo :) It seems to be generic algorithm to get phys_base for all 
platforms:
- on older platforms stolen_region points to system memory which starts 
at 0,
- on DG2 it uses lmem region which starts at 0 as well,
- on MTL stolen_region points to stolen-local which starts at 0x800000.

So this whole "if (IS_DGFX(i915)) {...} else {...}" could be replaced
with sth generic.
1. Find pte.
2. if(IS_DGFX(i915) && pte & GEN12_GGTT_PTE_LM) mem = 
i915->mm.regions[INTEL_REGION_LMEM_0] else mem = i915->mm.stolen_region
3. phys_base = (pte & I915_GTT_PAGE_MASK) - mem->region.start;

Regards
Andrzej


> Based on what I wrote before, I think `phys_base` is named incorrectly and
> that it does not reflect the physical address, but the start offset of
> i915->mm.stolen_region. So if we offset the start value of the stolen
> region, this code looks correct to me (and it also works on my
> MeteorLake device).
>
> What do you think?
>
>
> Many thanks,
> Paz