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diff --git a/Documentation/fb/deferred_io.rst b/Documentation/fb/deferred_io.rst new file mode 100644 index 000000000000..7300cff255a3 --- /dev/null +++ b/Documentation/fb/deferred_io.rst @@ -0,0 +1,79 @@ +=========== +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); |