Subversion Repositories or1k_soc_on_altera_embedded_dev_kit

Deferred IO

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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 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 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)

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No changes needed. mmap the framebuffer like normal and just use it.

How to use it: (for fbdev drivers)

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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);