[PATCH 0/4] Cursor's update inside kernel only

[olafBuddenhagen at gmx.net]

Hi,

On Mon, Jan 05, 2009 at 06:55:50PM -0200, Tiago Vignatti wrote:

> Under KMS, we can build a feature to update the cursor directly to
> screen without the continuous intervention of the userspace
> application (X server, wayland, etc). It's a fastpath for DRM based
> cursors obtained by short-circuit the kernel input layer and DRM
> module. This would solve all cursor's latency issues that we have in
> our current model [0].

IHMO punting and pushing this into the kernel in totally the wrong
approach. Following this method, you will end up running *everything* in
kernel space. (Hi, DOS!)

Rather, you should try to figure out *why* the update in user space has
latencies not present when doing it in kernel space. Is it because all
kernel memory is pinned? (Not true on all systems, BTW.) Is it because
event delivery between kernel space and user space triggers bad CPU
scheduling? Is it because the handling in user space is not properly
prioritized?

Once you know the real caus(es), you can go about fixing them --
adapting the design of the user space part, and possibly creating now
kernel interfaces is necessary.

Ultimately, the kernel/user space distinction is just an address space
boundary. There is no fundamental reason why that should cause any
latencies.

-antrik-