Miguel Ojeda wrote:
[...]
In this case, if nothing was ever written to the display, the CPU usage
would be _zero_ (as it should), and it would work nicely with dynticks
and such.
Hum, interesting. But what happens if...
1. For example, the userspace app maps the fb (nopage is called, dirty
flag set, timer created), and then the app doesn't use the fb. The
timer will refresh the LCD, without anything new to refresh, so we are
wasting CPU time. After refreshing, the driver clears the dirty flag
and unmap the buffer.
2. After some seconds, the userspace app decides to write the buffer.
As it is not mmaped anymore, it will simply get a segmentation fault,
right? AFAIK nopage() is not called again because we have destroyed
the mmapping information, and Linux can't know what are the nopage()
ops to call to.
No, this is not the sequence I thought of at all. I don't remember the
exact API functions you need to call (I have to read LDD3 again ;), but
if the hardware supports it, there must be a way. The plan is something
like:
- at mmap time you return a pointer to something that is not actually
mapped, and do nothing else
- when userspace actually writes to that area, you get a page fault,
and nopage is called. At this point you map the page, and set the dirty
state. All other writes from userspace until the timer completes are
done without faulting.
- when the timer completes, you unmap the page so that the next access
will generate a fault again
As I said, I don't remember the exact details, but this should be doable.
--
Paulo Marques - www.grupopie.com
"The face of a child can say it all, especially the
mouth part of the face."
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