drm/amdgpu/gmc: Fix AMDGPU_GART_PLACEMENT_LOW to not overlap with VRAM

[ Upstream commit 36d65da ]

When the GART placement is set to AMDGPU_GART_PLACEMENT_LOW:
Make sure that GART does not overlap with VRAM when
VRAM is configured to be in the low address space.

Solve this according to the following logic:
- When GART fits before VRAM, use zero address for GART
- Otherwise, put GART after the end of VRAM, aligned to 4 GiB

Previously, I had assumed this was not possible
so it was OK to not handle it, but now we got a report
from a user who has a board that is configured this way.

Fixes: 917f91d ("drm/amdgpu/gmc: add a way to force a particular placement for GART")
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 3d9de5d)
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: Timur Kristóf

drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.c

@@ -297,7 +297,10 @@ void amdgpu_gmc_gart_location(struct amdgpu_device *adev, struct amdgpu_gmc *mc,
 		mc->gart_start = max_mc_address - mc->gart_size + 1;
 		break;
 	case AMDGPU_GART_PLACEMENT_LOW:
-		mc->gart_start = 0;
+		if (size_bf >= mc->gart_size)
+			mc->gart_start = 0;
+		else
+			mc->gart_start = ALIGN(mc->fb_end, four_gb);
 		break;
 	case AMDGPU_GART_PLACEMENT_BEST_FIT:
 	default: