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