[Intel-gfx] [RFC PATCH 118/162] drm/i915/dg1: Reserve first 1MB of local memory

[Chris Wilson]

Quoting Matthew Auld (2020-11-30 11:09:57)
> On 27/11/2020 13:52, Chris Wilson wrote:
> > Quoting Matthew Auld (2020-11-27 12:06:34)
> >> From: Imre Deak <imre.deak at intel.com>
> >>
> >> On DG1 A0/B0 steppings the first 1MB of local memory must be reserved.
> >> One reason for this is that the 0xA0000-0xB0000 range is not accessible
> >> by the display, probably since this region is redirected to another
> >> memory location for legacy VGA compatibility.
> >>
> >> BSpec: 50586
> >> Testcase: igt/kms_big_fb/linear-64bpp-rotate-0
> >> Signed-off-by: Imre Deak <imre.deak at intel.com>
> >> ---
> >>   drivers/gpu/drm/i915/intel_region_lmem.c | 52 ++++++++++++++++++++++++
> >>   1 file changed, 52 insertions(+)
> >>
> >> diff --git a/drivers/gpu/drm/i915/intel_region_lmem.c b/drivers/gpu/drm/i915/intel_region_lmem.c
> >> index 939cf0d195a5..eafef7034680 100644
> >> --- a/drivers/gpu/drm/i915/intel_region_lmem.c
> >> +++ b/drivers/gpu/drm/i915/intel_region_lmem.c
> >> @@ -137,6 +137,48 @@ intel_setup_fake_lmem(struct drm_i915_private *i915)
> >>          return mem;
> >>   }
> >>   
> >> +static void get_legacy_lowmem_region(struct intel_uncore *uncore,
> >> +                                    u64 *start, u32 *size)
> >> +{
> >> +       *start = 0;
> >> +       *size = 0;
> >> +
> >> +       if (!IS_DG1_REVID(uncore->i915, DG1_REVID_A0, DG1_REVID_B0))
> >> +               return;
> >> +
> >> +       *size = SZ_1M;
> >> +
> >> +       DRM_DEBUG_DRIVER("LMEM: reserved legacy low-memory [0x%llx-0x%llx]\n",
> >> +                        *start, *start + *size);
> >> +}
> >> +
> >> +static int reserve_lowmem_region(struct intel_uncore *uncore,
> >> +                                struct intel_memory_region *mem)
> >> +{
> >> +       u64 reserve_start;
> >> +       u64 reserve_end;
> >> +       u64 region_start;
> >> +       u32 region_size;
> >> +       int ret;
> >> +
> >> +       get_legacy_lowmem_region(uncore, &region_start, &region_size);
> >> +       reserve_start = region_start;
> >> +       reserve_end = region_start + region_size;
> >> +
> >> +       if (!reserve_end)
> >> +               return 0;
> >> +
> >> +       DRM_INFO("LMEM: reserving low-memory region [0x%llx-0x%llx]\n",
> >> +                reserve_start, reserve_end);
> >> +       ret = i915_buddy_alloc_range(&mem->mm, &mem->reserved,
> >> +                                    reserve_start,
> >> +                                    reserve_end - reserve_start);
> > 
> > Isn't this now relative to the stolen offset? Should this be reserved,
> > or excluded like stolen?
> 
> AFAIK stolen is just snipped off at the end of lmem, so I don't think it 
> really matters if we exclude or reserve.

Right, misread, thought it was moving the start point.

> But for this if we exclude then 
> the region.start might have "strange" alignment, which is annoying since 
> alloc(some_power_of_two) might not give us the expected alignment, 
> whereas if we reserve then the allocator is aware, and so we should get 
> the proper alignment. Maybe you have better ideas with how to handle 
> this, but I think keeping the alignment property is nice.

The only tweak I would look at is making this reservation be the
property of the VGA decode. But if this promises not to live into
production, kiss.
-Chris