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

On 12/26/2014 11:19 AM, Laurent Pinchart wrote:
> Hi Thierry,
> 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
>>> third.
>>> 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
>>> drivers/gpu/drm/Makefile.
>>> 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.

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.

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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