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: c575b7eeb89f ("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

diff --git a/drivers/gpu/drm/xen/xen_drm_front_gem.c b/drivers/gpu/drm/xen/xen_drm_front_gem.c
index d303a2e..53c376d 100644 (file)
--- a/drivers/gpu/drm/xen/xen_drm_front_gem.c
+++ b/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,