Re: [PATCH v2 5/8] drm/i915/dg1: Reserve first 1MB of local memory

[Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>]

Quoting Matthew Auld (2021-01-26 15:12:56)
> From: Imre Deak <imre.deak@xxxxxxxxx>
> 
> 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
> 
> v2:
> - Reserve the memory on B0 as well.
> 
> v3: replace DRM_DEBUG/DRM_ERROR with drm_dbg/drm_err
> 
> Signed-off-by: Imre Deak <imre.deak@xxxxxxxxx>
> Signed-off-by: Matthew Auld <matthew.auld@xxxxxxxxx>
> ---
>  drivers/gpu/drm/i915/gt/intel_region_lmem.c | 52 +++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_region_lmem.c b/drivers/gpu/drm/i915/gt/intel_region_lmem.c
> index 4da808e35ecb..3b66221abe01 100644
> --- a/drivers/gpu/drm/i915/gt/intel_region_lmem.c
> +++ b/drivers/gpu/drm/i915/gt/intel_region_lmem.c
> @@ -138,6 +138,48 @@ intel_gt_setup_fake_lmem(struct intel_gt *gt)
>         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_dbg(&uncore->i915->drm, "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;

I'll bite. Why the impedance mismatch with a function introduced for
exactly this purpose?

if (!legacy_lowmem_region(uncore, &start, &end))
	return 0;

-Chris
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