Deferred IO
  -----------
  
  Deferred IO is a way to delay and repurpose IO. It uses host memory as a
  buffer and the MMU pagefault as a pretrigger for when to perform the device
  IO. The following example may be a useful explanation of how one such setup
  works:
  
  - userspace app like Xfbdev mmaps framebuffer
  - deferred IO and driver sets up fault and page_mkwrite handlers
  - userspace app tries to write to mmaped vaddress
  - we get pagefault and reach fault handler
  - fault handler finds and returns physical page
  - we get page_mkwrite where we add this page to a list
  - schedule a workqueue task to be run after a delay
  - app continues writing to that page with no additional cost. this is
    the key benefit.
  - the workqueue task comes in and mkcleans the pages on the list, then
   completes the work associated with updating the framebuffer. this is
    the real work talking to the device.
  - app tries to write to the address (that has now been mkcleaned)
  - get pagefault and the above sequence occurs again
  
  As can be seen from above, one benefit is roughly to allow bursty framebuffer
  writes to occur at minimum cost. Then after some time when hopefully things
  have gone quiet, we go and really update the framebuffer which would be
  a relatively more expensive operation.
  
  For some types of nonvolatile high latency displays, the desired image is
  the final image rather than the intermediate stages which is why it's okay
  to not update for each write that is occurring.
  
  It may be the case that this is useful in other scenarios as well. Paul Mundt
  has mentioned a case where it is beneficial to use the page count to decide
  whether to coalesce and issue SG DMA or to do memory bursts.
  
  Another one may be if one has a device framebuffer that is in an usual format,
  say diagonally shifting RGB, this may then be a mechanism for you to allow
  apps to pretend to have a normal framebuffer but reswizzle for the device
  framebuffer at vsync time based on the touched pagelist.
  
  How to use it: (for applications)
  ---------------------------------
  No changes needed. mmap the framebuffer like normal and just use it.
  
  How to use it: (for fbdev drivers)
  ----------------------------------
  The following example may be helpful.
  
  1. Setup your structure. Eg:
  
  static struct fb_deferred_io hecubafb_defio = {
  	.delay		= HZ,
  	.deferred_io	= hecubafb_dpy_deferred_io,
  };
  
  The delay is the minimum delay between when the page_mkwrite trigger occurs
  and when the deferred_io callback is called. The deferred_io callback is
  explained below.
  
  2. Setup your deferred IO callback. Eg:
  static void hecubafb_dpy_deferred_io(struct fb_info *info,
  				struct list_head *pagelist)
  
  The deferred_io callback is where you would perform all your IO to the display
  device. You receive the pagelist which is the list of pages that were written
  to during the delay. You must not modify this list. This callback is called
  from a workqueue.
  
  3. Call init
  	info->fbdefio = &hecubafb_defio;
  	fb_deferred_io_init(info);
  
  4. Call cleanup
  	fb_deferred_io_cleanup(info);