drm/shmem-helpers: Allocate wc pages on x86

intel-gfx-ci realized that something is not quite coherent anymore on
some platforms for our i915+vgem tests, when I tried to switch vgem
over to shmem helpers.

After lots of head-scratching I realized that I've removed calls to
drm_clflush. And we need those. To make this a bit cleaner use the
same page allocation tooling as ttm, which does internally clflush
(and more, as neeeded on any platform instead of just the intel x86
cpus i915 can be combined with).

Unfortunately this doesn't exist on arm, or as a generic feature. For
that I think only the dma-api can get at wc memory reliably, so maybe
we'd need some kind of GFP_WC flag to do this properly.

v2: Add a TODO comment about what should be done to support this in
other places (Thomas)
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210812131412.2487363-3-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_gem_shmem_helper.c

@@ -10,6 +10,10 @@
 #include <linux/slab.h>
 #include <linux/vmalloc.h>
 
+#ifdef CONFIG_X86
+#include <asm/set_memory.h>
+#endif
+
 #include <drm/drm.h>
 #include <drm/drm_device.h>
 #include <drm/drm_drv.h>
@@ -162,6 +166,16 @@ static int drm_gem_shmem_get_pages_locked(struct drm_gem_shmem_object *shmem)
 		return PTR_ERR(pages);
 	}
 
+	/*
+	 * TODO: Allocating WC pages which are correctly flushed is only
+	 * supported on x86. Ideal solution would be a GFP_WC flag, which also
+	 * ttm_pool.c could use.
+	 */
+#ifdef CONFIG_X86
+	if (shmem->map_wc)
+		set_pages_array_wc(pages, obj->size >> PAGE_SHIFT);
+#endif
+
 	shmem->pages = pages;
 
 	return 0;
@@ -203,6 +217,11 @@ static void drm_gem_shmem_put_pages_locked(struct drm_gem_shmem_object *shmem)
 	if (--shmem->pages_use_count > 0)
 		return;
 
+#ifdef CONFIG_X86
+	if (shmem->map_wc)
+		set_pages_array_wb(shmem->pages, obj->size >> PAGE_SHIFT);
+#endif
+
 	drm_gem_put_pages(obj, shmem->pages,
 			  shmem->pages_mark_dirty_on_put,
 			  shmem->pages_mark_accessed_on_put);