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 explaination of how one such setup 7works: 8 9- userspace app like Xfbdev mmaps framebuffer 10- deferred IO and driver sets up nopage and page_mkwrite handlers 11- userspace app tries to write to mmaped vaddress 12- we get pagefault and reach nopage handler 13- nopage 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 occuring. 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