drm/amdgpu: Fix AMDGPU_GTT_MAX_TRANSFER_SIZE for non-4K systems

drm/amdgpu: Fix AMDGPU_GTT_MAX_TRANSFER_SIZE for non-4K systems

Running RCCL unit tests on a system with a 64K PAGE_SIZE triggers
the following warning and causes the test to terminate on latest
upstream kernel:

WARNING: drivers/gpu/drm/amd/amdgpu/amdgpu_object.c:1335 at
amdgpu_bo_release_notify+0x1bc/0x280 [amdgpu],
CPU#18: rccl-UnitTests/33151

Call trace:
amdgpu_bo_release_notify
ttm_bo_release
amdgpu_gem_object_free
drm_gem_object_free
amdgpu_bo_unref
amdgpu_bo_create
amdgpu_bo_create_user
amdgpu_gem_object_create
amdgpu_amdkfd_gpuvm_alloc_memory_of_gpu
kfd_ioctl_alloc_memory_of_gpu
kfd_ioctl
sys_ioctl

The warning is triggered because
amdgpu_ttm_next_clear_entity() returns NULL when a clear buffer
operation is requested. This happens because the GART window
allocation for the default_entity, clear_entity and move_entity
fails during initialization.

Commit [1] introduced separate GART windows for the
default_entity, clear_entity and move_entity of each SDMA
instance. Their sizes are derived from
AMDGPU_GTT_MAX_TRANSFER_SIZE, which is currently defined as 1024
pages. This implicitly assumes a 4K PAGE_SIZE, where 1024 pages
correspond to a 4MB transfer. On a 64K PAGE_SIZE system, however,
the same value expands to 64MB.

The default_entity and clear_entity each allocate one
AMDGPU_GTT_MAX_TRANSFER_SIZE GART window, while the move_entity
allocates two such windows. This results in 16MB of GART space
per SDMA instance on a 4K PAGE_SIZE system, but 256MB per SDMA
instance on a 64K PAGE_SIZE system.

On an MI210 system with five SDMA instances and a 512MB GART
aperture, the total GART space required becomes 1.25GB,
exceeding the available GART aperture. Consequently, GART window
allocation fails, amdgpu_ttm_next_clear_entity() returns NULL,
and the above warning is triggered.

Redefine AMDGPU_GTT_MAX_TRANSFER_SIZE in bytes instead of page
units. Where a page count is required, convert it using
PAGE_SHIFT. This preserves the existing 4MB transfer size across
all PAGE_SIZE configurations while keeping GART window
allocations within the available GART aperture.

[1] https://lore.kernel.org/all/20260408100327.1372-3-pierre-eric.pelloux-prayer@amd.com/#t

Closes: https://gitlab.freedesktop.org/drm/amd/-/work_items/5435
Fixes: 897ee11ec020 ("drm/amdgpu: create multiple clear/move ttm entities")
Signed-off-by: Donet Tom <donettom@linux.ibm.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 27213b776a666d3030de5acc3cd75278197b0494)
Cc: stable@vger.kernel.org