drm/gpusvm: Introduce devmem_only flag for allocation

This commit adds a new flag, devmem_only, to the drm_gpusvm structure. The
purpose of this flag is to ensure that the get_pages function allocates
memory exclusively from the device's memory. If the allocation from
device memory fails, the function will return an -EFAULT error.

Required for shared CPU and GPU atomics on certain devices.

v3:
 - s/vram_only/devmem_only/

Fixes: 99624bdff8 ("drm/gpusvm: Add support for GPU Shared Virtual Memory")
Cc: stable@vger.kernel.org
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250512135500.1405019-2-matthew.brost@intel.com

drivers/gpu/drm/drm_gpusvm.c

@@ -1454,6 +1454,11 @@ map_pages:
 				goto err_unmap;
 			}
 
+			if (ctx->devmem_only) {
+				err = -EFAULT;
+				goto err_unmap;
+			}
+
 			addr = dma_map_page(gpusvm->drm->dev,
 					    page, 0,
 					    PAGE_SIZE << order,

include/drm/drm_gpusvm.h

@@ -286,6 +286,7 @@ struct drm_gpusvm {
  * @in_notifier: entering from a MMU notifier
  * @read_only: operating on read-only memory
  * @devmem_possible: possible to use device memory
+ * @devmem_only: use only device memory
  *
  * Context that is DRM GPUSVM is operating in (i.e. user arguments).
  */
@@ -294,6 +295,7 @@ struct drm_gpusvm_ctx {
 	unsigned int in_notifier :1;
 	unsigned int read_only :1;
 	unsigned int devmem_possible :1;
+	unsigned int devmem_only :1;
 };
 
 int drm_gpusvm_init(struct drm_gpusvm *gpusvm,