[Intel-gfx] [PATCH] drm/i915: enable semaphores on gen6 if dmar is not active

[Andrew Lutomirski]

On Mon, Apr 2, 2012 at 11:48 AM, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> Inspired by the recent ppgtt regression report, where switching of
> dmar only for the gpu seems to fix things completely, I've looked
> again at the semaphores+vt-d situation.
>
> Contrary to my earlier testing a few months back my system is now
> stable with dmar disabled for the igd, and not only when disabling
> dmar completely.
>
> So I'm rather hopeful that all our recent fixes for snb have changed
> things for code and it's time to try enabling semaphores again. We've
> also had issues with enabling semaphores which are not vt-d related,
> but I guess these are all fixed by the autoreport-disabling and lazy
> request fix. And there's only one way to find out whether there are
> still other issues ...
>
> Signed-Off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
>
> ---
>
> If no further vt-d regressions show up in the 3.4 cycle I plan to
> merge this into -next for 3.5 (in a month or so). Comments about how
> unfeasibly you deem this highly welcome.
> -Daniel
> ---
>  drivers/gpu/drm/i915/i915_gem_execbuffer.c |    8 +++++---
>  1 files changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> index 8e0b686..ac52433 100644
> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> @@ -849,9 +849,11 @@ intel_enable_semaphores(struct drm_device *dev)
>        if (i915_semaphores >= 0)
>                return i915_semaphores;
>
> -       /* Disable semaphores on SNB */
> -       if (INTEL_INFO(dev)->gen == 6)
> -               return 0;
> +#ifdef CONFIG_INTEL_IOMMU
> +       /* Disable semaphores on SNB if VT-d is on. */
> +       if (INTEL_INFO(dev)->gen == 6 && intel_iommu_gfx_mapped)
> +               return false;

It might be nice to have a printk here giving instructions for how to work
around this.  For example:

i915: Disabling semaphores to work around a DMAR issue.  As an
alternative, boot with intel_iommu=igfx_off [or on, igfx_off, or
whatever it's supposed to be].

The documentation in kernel-parameters.txt is at best unclear to the
uninitiated.

--Andy