linux/Documentation/fb/deferred_io.txt
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   1Deferred IO
   2-----------
   3
   4Deferred IO is a way to delay and repurpose IO. It uses host memory as a
   5buffer and the MMU pagefault as a pretrigger for when to perform the device
   6IO. The following example may be a useful explanation of how one such setup
   7works:
   8
   9- userspace app like Xfbdev mmaps framebuffer
  10- deferred IO and driver sets up fault and page_mkwrite handlers
  11- userspace app tries to write to mmaped vaddress
  12- we get pagefault and reach fault handler
  13- fault handler finds and returns physical page
  14- we get page_mkwrite where we add this page to a list
  15- schedule a workqueue task to be run after a delay
  16- app continues writing to that page with no additional cost. this is
  17  the key benefit.
  18- the workqueue task comes in and mkcleans the pages on the list, then
  19 completes the work associated with updating the framebuffer. this is
  20  the real work talking to the device.
  21- app tries to write to the address (that has now been mkcleaned)
  22- get pagefault and the above sequence occurs again
  23
  24As can be seen from above, one benefit is roughly to allow bursty framebuffer
  25writes to occur at minimum cost. Then after some time when hopefully things
  26have gone quiet, we go and really update the framebuffer which would be
  27a relatively more expensive operation.
  28
  29For some types of nonvolatile high latency displays, the desired image is
  30the final image rather than the intermediate stages which is why it's okay
  31to not update for each write that is occurring.
  32
  33It may be the case that this is useful in other scenarios as well. Paul Mundt
  34has mentioned a case where it is beneficial to use the page count to decide
  35whether to coalesce and issue SG DMA or to do memory bursts.
  36
  37Another one may be if one has a device framebuffer that is in an usual format,
  38say diagonally shifting RGB, this may then be a mechanism for you to allow
  39apps to pretend to have a normal framebuffer but reswizzle for the device
  40framebuffer at vsync time based on the touched pagelist.
  41
  42How to use it: (for applications)
  43---------------------------------
  44No changes needed. mmap the framebuffer like normal and just use it.
  45
  46How to use it: (for fbdev drivers)
  47----------------------------------
  48The following example may be helpful.
  49
  501. Setup your structure. Eg:
  51
  52static struct fb_deferred_io hecubafb_defio = {
  53        .delay          = HZ,
  54        .deferred_io    = hecubafb_dpy_deferred_io,
  55};
  56
  57The delay is the minimum delay between when the page_mkwrite trigger occurs
  58and when the deferred_io callback is called. The deferred_io callback is
  59explained below.
  60
  612. Setup your deferred IO callback. Eg:
  62static void hecubafb_dpy_deferred_io(struct fb_info *info,
  63                                struct list_head *pagelist)
  64
  65The deferred_io callback is where you would perform all your IO to the display
  66device. You receive the pagelist which is the list of pages that were written
  67to during the delay. You must not modify this list. This callback is called
  68from a workqueue.
  69
  703. Call init
  71        info->fbdefio = &hecubafb_defio;
  72        fb_deferred_io_init(info);
  73
  744. Call cleanup
  75        fb_deferred_io_cleanup(info);
  76
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