Re: iommu_intel or i915 regression in 4.18, 4.19.12 and drm-tip

From: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
To: Eric Wong <e@80x24.org>
Cc: Daniel Vetter <daniel@ffwll.ch>, David Airlie <airlied@linux.ie>,
	David Woodhouse <dwmw2@infradead.org>,
	Jani Nikula <jani.nikula@linux.intel.com>,
	Joerg Roedel <joro@8bytes.org>,
	Rodrigo Vivi <rodrigo.vivi@intel.com>,
	iommu@lists.linux-foundation.org,
	intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
	linux-kernel@vger.kernel.org
Subject: Re: iommu_intel or i915 regression in 4.18, 4.19.12 and drm-tip
Date: Fri, 04 Jan 2019 10:39:23 +0200	[thread overview]
Message-ID: <154659116310.4596.13613897418163029789@jlahtine-desk.ger.corp.intel.com> (raw)
In-Reply-To: <20190104010626.e6yqdqkmdcqjepke@dcvr>

Quoting Eric Wong (2019-01-04 03:06:26)
> Joonas Lahtinen <joonas.lahtinen@linux.intel.com> wrote:
> > Quoting Eric Wong (2018-12-27 13:49:48)
> > > I just got a used Thinkpad X201 (Core i5 M 520, Intel QM57
> > > chipset) and hit some kernel panics while trying to view
> > > image/animation-intensive stuff in Firefox (X11) unless I use
> > > "iommu_intel=igfx_off".
> > > 
> > > With Debian stable backport kernels, "linux-image-4.17.0-0.bpo.3-amd64"
> > > (4.17.17-1~bpo9+1) has no problems.  But "linux-image-4.18.0-0.bpo.3-amd64"
> > > (4.18.20-2~bpo9+1) gives a blank screen before I can login via agetty
> > > and run startx.
> 
> > Most confusing about this is that 4.17 would have worked to begin with,
> > without intel_iommu=igfx_off (unless it was the default for older
> > kernel?)
> 
> Yeah, so the Debian bpo 4.17(.17) kernel did not set
> CONFIG_INTEL_IOMMU_DEFAULT_ON, so I didn't encounter problems.
> My self-built kernels all set CONFIG_INTEL_IOMMU_DEFAULT_ON.

So it's the case that IOMMU never worked on your machine.

My recommendation would be to simply use intel_iommu=igfx_off if you
need IOMMU.

Old hardware is known to have issues with IOMMU, and retroactively
enabling IOMMU on those machines just brings them up :/

Regards, Joonas

> Booting the Debian 4.17 kernel with "intel_iommu=on" gives the
> same hanging problem I hit with self-built 4.19.{12,13} kernels.
> 
> I'm not sure how far back the problem goes (maybe forever),
> since I only got this hardware.  Not sure what's the problem
> with Debian 4.18, either; but (self-built) 4.19.13 is fine w/o
> CONFIG_INTEL_IOMMU_DEFAULT_ON.
> 
> Debian backports doesn't have kernels for 4.19 or 4.20, yet.
> 
> > Did you maybe update other parts of the system while updating the
> > kernel?
> 
> Definitely not; just the kernel + headers ("make bindeb-pkg)".
> 
> > If you could attach full boot dmesg from working and non-working kernel +
> > have config file of both kernel's in Bugzilla. That'd be a good start!
> 
> Sorry, I get anxiety attacks when it comes to logins and forms.
> Anyways, I managed to get the Debian kernel dmesg output uploaded
> with and without iommu_intel=on:
> https://bugs.freedesktop.org/attachment.cgi?bugid=109219