drm/xen-front: Fix mmap attributes for display buffers

When GEM backing storage is allocated those are normal pages,
so there is no point using pgprot_writecombine while mmaping.
This fixes mismatch of buffer pages' memory attributes between
the frontend and backend which may cause screen artifacts.

Fixes: c575b7ee ("drm/xen-front: Add support for Xen PV display frontend")
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Suggested-by: Julien Grall <julien.grall@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190129150422.19867-1-andr2000@gmail.com

drivers/gpu/drm/xen/xen_drm_front_gem.c

@@ -235,8 +235,14 @@ static int gem_mmap_obj(struct xen_gem_object *xen_obj,
 	vma->vm_flags &= ~VM_PFNMAP;
 	vma->vm_flags |= VM_MIXEDMAP;
 	vma->vm_pgoff = 0;
-	vma->vm_page_prot =
-			pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
+	/*
+	 * According to Xen on ARM ABI (xen/include/public/arch-arm.h):
+	 * all memory which is shared with other entities in the system
+	 * (including the hypervisor and other guests) must reside in memory
+	 * which is mapped as Normal Inner Write-Back Outer Write-Back
+	 * Inner-Shareable.
+	 */
+	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
 
 	/*
 	 * vm_operations_struct.fault handler will be called if CPU access
@@ -282,8 +288,9 @@ void *xen_drm_front_gem_prime_vmap(struct drm_gem_object *gem_obj)
 	if (!xen_obj->pages)
 		return NULL;
 
+	/* Please see comment in gem_mmap_obj on mapping and attributes. */
 	return vmap(xen_obj->pages, xen_obj->num_pages,
-		    VM_MAP, pgprot_writecombine(PAGE_KERNEL));
+		    VM_MAP, PAGE_KERNEL);
 }
 
 void xen_drm_front_gem_prime_vunmap(struct drm_gem_object *gem_obj,