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

[Andrew Lutomirski]

On Mon, Apr 2, 2012 at 1:52 PM, Daniel Vetter <daniel at ffwll.ch> wrote:
> On Mon, Apr 02, 2012 at 01:45:39PM -0700, Andrew Lutomirski wrote:
>> 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.
>
> If this really does work, I'll look into this. Atm it's still unclear
> where to exactly put the plain, we need to annoy internal hardware people
> to analyze this. Once we have enough evidence that the combination of gpu
> with various features enable plus VT-d just doesn't seem to work reliably.
>
> So can you check whether things do indeed work with this patch, atop
> kernel 3.4-rc1? Iirc you've been the one with the most trouble when
> semaphores are enabled ...

I've had a hard time reproducing the problems lately.  The
unconditional instant hard hang went away a few months ago, I think.
I'll give it another shot when I get home to that machine.

--Andy