1 ===========
2 Deferred IO
3 ===========
4
5 Deferred IO is a way to delay and repurpose IO. It uses host memory as a
6 buffer and the MMU pagefault as a pretrigger for when to perform the device
7 IO. The following example may be a useful explanation of how one such setup
8 works:
9
10 - userspace app like Xfbdev mmaps framebuffer
11 - deferred IO and driver sets up fault and page_mkwrite handlers
12 - userspace app tries to write to mmapped vaddress
13 - we get pagefault and reach fault handler
14 - fault handler finds and returns physical page
15 - we get page_mkwrite where we add this page to a list
16 - schedule a workqueue task to be run after a delay
17 - app continues writing to that page with no additional cost. this is
18 the key benefit.
19 - the workqueue task comes in and mkcleans the pages on the list, then
20 completes the work associated with updating the framebuffer. this is
21 the real work talking to the device.
22 - app tries to write to the address (that has now been mkcleaned)
23 - get pagefault and the above sequence occurs again
24
25 As can be seen from above, one benefit is roughly to allow bursty framebuffer
26 writes to occur at minimum cost. Then after some time when hopefully things
27 have gone quiet, we go and really update the framebuffer which would be
28 a relatively more expensive operation.
29
30 For some types of nonvolatile high latency displays, the desired image is
31 the final image rather than the intermediate stages which is why it's okay
32 to not update for each write that is occurring.
33
34 It may be the case that this is useful in other scenarios as well. Paul Mundt
35 has mentioned a case where it is beneficial to use the page count to decide
36 whether to coalesce and issue SG DMA or to do memory bursts.
37
38 Another one may be if one has a device framebuffer that is in an usual format,
39 say diagonally shifting RGB, this may then be a mechanism for you to allow
40 apps to pretend to have a normal framebuffer but reswizzle for the device
41 framebuffer at vsync time based on the touched pagelist.
42
43 How to use it: (for applications)
44 ---------------------------------
45 No changes needed. mmap the framebuffer like normal and just use it.
46
47 How to use it: (for fbdev drivers)
48 ----------------------------------
49 The following example may be helpful.
50
51 1. Setup your structure. Eg::
52
53 static struct fb_deferred_io hecubafb_defio = {
54 .delay = HZ,
55 .deferred_io = hecubafb_dpy_deferred_io,
56 };
57
58 The delay is the minimum delay between when the page_mkwrite trigger occurs
59 and when the deferred_io callback is called. The deferred_io callback is
60 explained below.
61
62 2. Setup your deferred IO callback. Eg::
63
64 static void hecubafb_dpy_deferred_io(struct fb_info *info,
65 struct list_head *pagelist)
66
67 The deferred_io callback is where you would perform all your IO to the display
68 device. You receive the pagelist which is the list of pages that were written
69 to during the delay. You must not modify this list. This callback is called
70 from a workqueue.
71
72 3. Call init::
73
74 info->fbdefio = &hecubafb_defio;
75 fb_deferred_io_init(info);
76
77 4. Call cleanup::
78
79 fb_deferred_io_cleanup(info);
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