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author | Jaya Kumar <jayakumar.lkml@gmail.com> | 2007-05-08 00:37:37 -0700 |
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committer | Linus Torvalds <torvalds@woody.linux-foundation.org> | 2007-05-08 11:15:26 -0700 |
commit | 60b59beafba875aef6d378078bce0baf2287ae14 (patch) | |
tree | bb599c0e2ad43ee8f515a9f9af009442931b6a37 /Documentation | |
parent | 3a2842480bbef42c3c90e14c1f378360d8c20a0c (diff) | |
download | kernel_samsung_aries-60b59beafba875aef6d378078bce0baf2287ae14.zip kernel_samsung_aries-60b59beafba875aef6d378078bce0baf2287ae14.tar.gz kernel_samsung_aries-60b59beafba875aef6d378078bce0baf2287ae14.tar.bz2 |
fbdev: mm: Deferred IO support
This implements deferred IO support in fbdev. Deferred IO is a way to delay
and repurpose IO. This implementation is done using mm's page_mkwrite and
page_mkclean hooks in order to detect, delay and then rewrite IO. This
functionality is used by hecubafb.
[adaplas]
This is useful for graphics hardware with no directly addressable/mappable
framebuffer. Implementing this will allow the "framebuffer" to be accesible
from user space via mmap().
Signed-off-by: Jaya Kumar <jayakumar.lkml@gmail.com>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation')
-rw-r--r-- | Documentation/fb/deferred_io.txt | 75 |
1 files changed, 75 insertions, 0 deletions
diff --git a/Documentation/fb/deferred_io.txt b/Documentation/fb/deferred_io.txt new file mode 100644 index 0000000..73cf9fb --- /dev/null +++ b/Documentation/fb/deferred_io.txt @@ -0,0 +1,75 @@ +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 explaination of how one such setup +works: + +- userspace app like Xfbdev mmaps framebuffer +- deferred IO and driver sets up nopage and page_mkwrite handlers +- userspace app tries to write to mmaped vaddress +- we get pagefault and reach nopage handler +- nopage 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 occuring. + +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); |