76 lines
3 KiB
Plaintext
76 lines
3 KiB
Plaintext
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);
|