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drm/amdgpu: cap GTT size to physical RAM on APUs

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On APUs, the GTT pool is backed by system RAM, but its size is not bound
to the non-carveout memory that actually backs it. A user can end up
with GTT + VRAM exceeding total physical memory through the following
sequence:

 - Have a large non-carveout memory space (~128GB) and accordingly set a
   large GTT (~100GB) via the ttm module parameter.
 - Lower the non-carveout memory space in BIOS by increasing the UMA
   Frame Buffer Size (VRAM) to 64GB.
 - The previously set GTT value (~100GB) persists, even though the new
   non-carveout space (64GB) can no longer back it.

This leads to a case where kernel reports GTT (100GB) + VRAM (64GB)
despite the sum being greater than total physical memory (128GB).

Cap the GTT size to totalram_pages() on APUs. totalram_pages() already
excludes the VRAM carveout, so the resulting GTT can never exceed the
system RAM that actually backs it.

Signed-off-by: Harkirat Gill <harkirat.gill@amd.com>
Reviewed-by: David Francis <David.Francis@amd.com>
Assisted-by: Claude:claude-opus-4
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
This commit is contained in:
Harkirat Gill
2026-07-28 19:17:33 -04:00
committed by Alex Deucher
parent 757ba0790b
commit 5dafdd6492
1 changed files with 12 additions and 0 deletions
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drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
+12
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@@ -2174,6 +2174,18 @@ int amdgpu_ttm_init(struct amdgpu_device *adev)
gtt_size = configured_size;
}
/* Cap GTT so that it does not exceed total physical RAM. */
if (adev->flags & AMD_IS_APU) {
u64 phys_ram = (u64)totalram_pages() << PAGE_SHIFT;
if (gtt_size > phys_ram) {
gtt_size = phys_ram;
dev_info(adev->dev,
"Capping GTT to %uM to not exceed available system memory\n",
(unsigned int)(gtt_size / (1024 * 1024)));
}
}
/* Initialize GTT memory pool */
r = amdgpu_gtt_mgr_init(adev, gtt_size);
if (r) {