Re: [PATCH] drm: Compare only lower 32 bits of framebuffer map offsets

[Dave Airlie <airlied@redhat.com>]

On Mon, 2011-05-30 at 01:12 +0200, Tormod Volden wrote:
> From: Tormod Volden <debian.tormod@gmail.com>
> 
> Drivers using multiple framebuffers got broken by commit
> 41c2e75e60200a860a74b7c84a6375c105e7437f which ignored the framebuffer
> (or register) map offset when looking for existing maps. The rationale
> was that the kernel-userspace ABI is fixed at a 32-bit offset, so the
> real offsets could not always be handed over for comparison.
> 
> Instead of ignoring the offset we will compare the lower 32 bit. Drivers
> using multiple framebuffers should just make sure that the lower 32 bit
> are different. The existing drivers in question are practically limited
> to 32-bit systems so that should be fine for them.
> 
> It is assumed that current drivers always specify a correct framebuffer
> map offset, even if this offset was ignored since above commit. So this
> patch should not change anything for drivers using only one framebuffer.
> 
> Drivers needing multiple framebuffers with 64-bit map offsets will need
> to cook up something, for instance keeping an ID in the lower bits,
> which is to be aligned away when it comes to using the offset.
> 
> Signed-off-by: Tormod Volden <debian.tormod@gmail.com>
> ---
> 
> What about this idea? (Untested patch)

If you test it and it works I like it best. Simple and clear, and pretty
close to what I was thinking was a good idea.

As you say if someone needs this functionality in a new driver they can
fix it, but really new drivers shouldn't be doing anything in this area.

Dave.