amd-iommu: can't boot with amdgpu, AMD-Vi: Completion-Wait loop timed out

[joro at 8bytes.org]

On Fri, Mar 17, 2017 at 11:53:09AM -0400, Alex Deucher wrote:
> On Fri, Mar 17, 2017 at 8:15 AM, Daniel Drake <drake at endlessm.com> wrote:
> > Hi,
> >
> > On Mon, Mar 13, 2017 at 2:01 PM, Deucher, Alexander
> > <Alexander.Deucher at amd.com> wrote:
> >> > We are unable to boot Acer Aspire E5-553G (AMD FX-9800P RADEON R7) nor
> >> > Acer Aspire E5-523 with standard configurations because during boot
> >> > the screen is flooded with the following error message over and over:
> >> >
> >> >   AMD-Vi: Completion-Wait loop timed out
> >>
> >> We ran into similar issues and bisected it to commit b1516a14657acf81a587e9a6e733a881625eee53.  I'm not too familiar with the IOMMU hardware to know if this is an iommu or display driver issue yet.
> >
> > We can confirm that reverting this commit solves the issue.
> >
> > Given that that commit is an optimization, but it has introduced a
> > regression on multiple platforms, and has been like this for 8 months,
> > it would be common practice to now revert this patch upstream until
> > the regression is fixed. Could you please send a new patch to do this?
> >
> > Also, we would be happy to test any real solutions to this issue while
> > we still have the affected units in hand.
> 
> No objections to a revert here.

Big objection here. Since this only happens with amdgpu so far we
shouldn't rule out a display-driver issue.

Reverting that patch basically destroys iommu-performance on AMD
systems. Doing this for all devices just to make amdgpu working is
overkill at this stage of the debugging.



	Joerg