Re: [PATCH 0/2] Change order of linkage in kernel makefiles for amdkfd

From: Christian König
Date: Mon Dec 29 2014 - 06:09:40 EST

Am 29.12.2014 um 09:16 schrieb Laurent Pinchart:
Hi Oded,

On Sunday 28 December 2014 13:36:50 Oded Gabbay wrote:
On 12/26/2014 11:19 AM, Laurent Pinchart wrote:
On Thursday 25 December 2014 14:20:59 Thierry Reding wrote:
On Mon, Dec 22, 2014 at 01:07:13PM +0200, Oded Gabbay wrote:
This small patch-set, was created to solve the bug described at (Kernel panic when
trying use amdkfd driver on Kaveri). It replaces the previous patch-set
called [PATCH 0/3] Use workqueue for device init in amdkfd

That bug appears only when radeon, amdkfd and amd_iommu_v2 are compiled
inside the kernel (not as modules). In that case, the correct loading
order, as determined by the exported symbol used by each driver, is
not enforced anymore and the kernel loads them based on who was linked
first. That makes radeon load first, amdkfd second and amd_iommu_v2

Because the initialization of a device in amdkfd is initiated by radeon,
and can only be completed if amdkfd and amd_iommu_v2 were loaded and
initialized, then in the case mentioned above, this initalization fails
and there is a kernel panic as some pointers are not initialized but
used nontheless.

To solve this bug, this patch-set moves iommu/ before gpu/ in
drivers/Makefile and also moves amdkfd/ before radeon/ in

The rationale is that in general, AMD GPU devices are dependent on AMD
IOMMU controller functionality to allow the GPU to access a process's
virtual memory address space, without the need for pinning the memory.
That's why it makes sense to initialize the iommu/ subsystem ahead of
the gpu/ subsystem.
I strongly object to this patch set. This makes assumptions about how
the build system influences probe order. That's bad because seemingly
unrelated changes could easily break this in the future.

We already have ways to solve this kind of dependency (driver probe
deferral), and I think you should be using it to solve this particular
problem rather than some linking order hack.
While I agree with you that probe deferral is the way to go, I believe
linkage ordering can still be used as an optimization to avoid deferring
probe in the most common cases. I'm thus not opposed to moving iommu/
earlier in link order (provided we can properly test for side effects, as
the jump is pretty large), but not as a replacement for probe deferral.
My thoughts exactly. If this was some extreme use case, than it would be
justified to solve it with probe deferral. But I think that for most common
cases, GPU are dependent on IOMMU and *not* vice-versa.

Fixing this through deferred probing sounds like the correct long term solution to me as well.

But what Thierry is referring to here is probably the approach of returning -EAGAIN from the probe method (at least that was the last status when I looked into this).

The problem with this approach is the interface design between radeon and amdkfd. amdkfd simply doesn't have a probe method which gets called when the hardware is detected and can return -EAGAIN. Instead amdkfd is called by radeon after hardware initialization when it is way to late for such a thing.

BTW, my first try at solving this was to use probe deferral (using
workqueue), but the feedback I got from Christian and Dave was that moving
iommu/ linkage before gpu/ was a much more simpler solution.
To clarify my position, I believe changing the link order can be a worthwhile
optimization, but I'm uncertain about the long term viability of that change
as a fix. Probe deferral has been introduced because not all probe ordering
issues can be fixed through link ordering, so we should fix the problem

This being said, if modifying the link order can help for now without
introducing negative side effects, it would only postpone the real fix, so I'm
not opposed to it.

Yeah, that sounds like the right approach to me as well. In general I would prefer that modules compiled into the kernel load by the order of their symbol dependency just like standalone modules do.

That's what Rusty proposed more than 10 years ago when he reworked the module system and I'm actually not sure why it was never done this way. I can only find the initial patch to do so in the mail history, but not why it was rejected.


In addition, Linus said he doesn't object to this "band-aid". See:


Coincidentally there's a separate thread currently going on that deals
with IOMMUs and probe order. The solution being worked on is currently
somewhat ARM-specific, so adding a couple of folks for visibility. It
looks like we're going to need something more generic since this is a
problem that even the "big" architectures need to solve.

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