[Intel-gfx] [PATCH] drmmode: unreserve userspace fences

[Eric Anholt]

On Sun, 31 Jan 2010 11:59:56 +0100, Daniel Vetter <daniel at ffwll.ch> wrote:
> Hi Eric,
> 
> I've justed noticed that you pushed a patch to fix the kernel. But just
> patching the kernel is not enough because new userspaces is still broken
> for older kernels. And currently "older kernels" means _any_ released
> mainline kernel.
> 
> Without this patch I'm experiencing unnecessary uxa fallbacks (due to
> failed execbuf ioctls due to fence starvation).
> 
> Please reconsider applying.

Unless required, we try to avoid adding workarounds in one part of our
driver for bugs in other parts of our driver.  Given that the kernel
backport patch is smaller than the workaround patch, I think the
solution is obvious: test and send the kernel fix to stable at kernel.org.

If you've got failed execbuf due to fence starvation, then something
else is wrong, because that should never happen unless there just aren't
enough fences for the number of tiled buffers in one single operation.
Looks like libdrm is using the NUM_FENCES_AVAIL as how many fences it
can use, but there are pinned scanout buffers holding onto fences so the
number of available fences for an execbuf is actually less than that.
Maybe NUM_FENCES_AVAIL should report (num_fences - start_fence_reg -
num_active_scanout_fences) or (num_fences - start_fence_reg -
possible_active_scanout_fences).
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