[RFC PATCH] Add XMir Xorg-nested-in-Mir helper extension

[Chris Wilson]

On Mon, Jul 22, 2013 at 03:08:02PM +1000, christopher.halse.rogers at canonical.com wrote:
> From: Christopher James Halse Rogers <raof at ubuntu.com>
> 
> This is missing too much functionality to be usefully appiled, but the
> skeleton is here and the APIs it relies on are sufficiently stable. Sending
> to the list for extra visibility and for preliminary comments.

So one comment that came up, that could do with a few wiser heads, was
the rationale for the buffer_available callback. This callback is sent
to the driver from inside a socket handler after XMir receives
notification from the mir-system-compositor thread that the fd is
available for redrawing. The intent is that this provides a hook for
the driver to submit fresh rendering as early as possible in order for
the display to continue to update (in its psuedo-frontbuffer fashion)
without waiting upon clients to become idle. The flaw in this argument,
is that the buffer_available socket is only checked for readibility when
the server finishes processing all client requests, i.e. at the same time
as the normal blockhandler would run.

The current solution to this problem on the input side is to use
signals. Another would be to use threads (and then make the rendering
code threadsafe, which is quite unpalatable). However, it also looks
like we can use "dispatchException |= DE_PRIORITYCHANGE;" to interrupt
dispatch processing and service the render request. That would then
require making dispatchException threadsafe (using atomic bit operations
should suffice).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre