Export fb_deferred_io_mmap so drivers can change vma->vm_page_prot.
When the framebuffer memory is allocated using dma_alloc_writecombine()
instead of vmalloc(), I get cache syncing problems on ARM.
This solves it:

static int drm_fbdev_cma_deferred_io_mmap(struct fb_info *info,
					  struct vm_area_struct *vma)
{
	fb_deferred_io_mmap(info, vma);
	vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);

	return 0;
}

Could this have been done in the core?
Drivers that don't set (struct fb_ops *)->fb_mmap, gets a call to
fb_pgprotect() at the end of the default fb_mmap implementation
(drivers/video/fbdev/core/fbmem.c). This is an architecture specific
function that on many platforms uses pgprot_writecombine(), but not on
all. And looking at some of the fb_mmap implementations, some of them
sets vm_page_prot to nocache for instance, so I think the safest bet is
to do this in the driver and not in the fbdev core. And we can't call
fb_pgprotect() from fb_deferred_io_mmap() either because we don't have
access to the file pointer that powerpc needs.

Signed-off-by: Noralf Trønnes <redacted>
---

Changes since v1:
- Expand commit message

 drivers/video/fbdev/core/fb_defio.c | 3 ++-
 include/linux/fb.h                  | 1 +
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
index 57721c7..74b5bca 100644
--- a/drivers/video/fbdev/core/fb_defio.c
+++ b/drivers/video/fbdev/core/fb_defio.c

@@ -164,7 +164,7 @@ static const struct address_space_operations fb_deferred_io_aops = {
 	.set_page_dirty = fb_deferred_io_set_page_dirty,
 };

-static int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma)
+int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma)
 {
 	vma->vm_ops = &fb_deferred_io_vm_ops;
 	vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;

@@ -173,6 +173,7 @@ static int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma)
 	vma->vm_private_data = info;
 	return 0;
 }
+EXPORT_SYMBOL(fb_deferred_io_mmap);

 /* workqueue callback */
 static void fb_deferred_io_work(struct work_struct *work)

diff --git a/include/linux/fb.h b/include/linux/fb.h
index dfe8835..a964d07 100644
--- a/include/linux/fb.h
+++ b/include/linux/fb.h

@@ -673,6 +673,7 @@ static inline void __fb_pad_aligned_buffer(u8 *dst, u32 d_pitch,
 }

 /* drivers/video/fb_defio.c */
+int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma);
 extern void fb_deferred_io_init(struct fb_info *info);
 extern void fb_deferred_io_open(struct fb_info *info,
 				struct inode *inode,
--

2.2.2