drm/fb_cma_helper: Implement fb_mmap callback

In the absence of an fb_mmap callback, the fbdev code falls back to a
naive implementation which relies upon the DMA address being the same
as the physical address, and the buffer being physically contiguous
from there. Whilst this often holds for standard CMA allocations via
the platform's regular DMA ops, if the allocation is provided by an
IOMMU then such assumptions can fall apart spectacularly.

To resolve this, reroute the fb_mmap call to the appropriate DMA API
implementation, as per the other cma_helper calls.
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/8fd95ac1440e0f01daad6d4380be3a4c8fa61055.1465301219.git.robin.murphy@arm.com

drivers/gpu/drm/drm_fb_cma_helper.c

@@ -23,6 +23,7 @@
 #include <drm/drm_crtc_helper.h>
 #include <drm/drm_gem_cma_helper.h>
 #include <drm/drm_fb_cma_helper.h>
+#include <linux/dma-mapping.h>
 #include <linux/module.h>
 
 #define DEFAULT_FBDEFIO_DELAY_MS 50
@@ -302,6 +303,12 @@ int drm_fb_cma_debugfs_show(struct seq_file *m, void *arg)
 EXPORT_SYMBOL_GPL(drm_fb_cma_debugfs_show);
 #endif
 
+static int drm_fb_cma_mmap(struct fb_info *info, struct vm_area_struct *vma)
+{
+	return dma_mmap_writecombine(info->device, vma, info->screen_base,
+				     info->fix.smem_start, info->fix.smem_len);
+}
+
 static struct fb_ops drm_fbdev_cma_ops = {
 	.owner		= THIS_MODULE,
 	.fb_fillrect	= drm_fb_helper_sys_fillrect,
@@ -312,6 +319,7 @@ static struct fb_ops drm_fbdev_cma_ops = {
 	.fb_blank	= drm_fb_helper_blank,
 	.fb_pan_display	= drm_fb_helper_pan_display,
 	.fb_setcmap	= drm_fb_helper_setcmap,
+	.fb_mmap	= drm_fb_cma_mmap,
 };
 
 static int drm_fbdev_cma_deferred_io_mmap(struct fb_info *info,