Re: [PATCH 1/3] amdkfd: Don't clear *kfd2kgd on kfd_module_init

[Oded Gabbay]

On 12/21/2014 02:19 PM, Christian KÃnig wrote:
> Am 21.12.2014 um 12:34 schrieb Oded Gabbay:
> > On 12/21/2014 01:27 PM, Christian KÃnig wrote:
> > > Am 20.12.2014 um 21:46 schrieb Oded Gabbay:
> > > > When amdkfd and radeon are compiled inside the kernel image (not as modules),
> > > > radeon will load before amdkfd and will set *kfd2kgd to its interface
> > > > structure. Therefore, we must not set *kfd2kgd to NULL when amdkfd is loaded
> > > > because it will override radeon's initialization and cause kernel BUG.
> > > >
> > > > Signed-off-by: Oded Gabbay <oded.gabbay@xxxxxxx>
> > >
> > > You should probably rather fix the dependency between the two modules to get an
> > > determined load order instead of doing such nasty workarounds.
> > >
> > > Christian.
> >
> > The problem is that when modules are compiled inside the kernel, there is NO
> > determined load order and there is no mechanism to enforce that. If there
> > is/was such a mechanism, I would of course prefer to use it.
>
> There should be an determined order based on the symbol use, otherwise
> initializing most of the kernel modules wouldn't work as expected. For example
> radeon depends on the drm module must be loaded before radeon is loaded.
There should be, but when the modules are compiled in, they are loaded based on 
link order only, if they are in the same group, and the groups are loaded by a 
pre-defined order.
The groups are: pure, core, postcore, arch, subsys, fs, device (which represents 
all the modules) and late. See init.h

So radeon, amdkfd and amd_iommu_v2 are all in device group, and in the group 
they are ordered by their link order.

Yes, radeon loads after drm, because drm*.o are before radeon*.o in the 
Makefile. See 
http://stackoverflow.com/questions/5669647/linux-order-of-statically-linked-module-loading


> > Actually, I don't understand why the kernel doesn't enforce the order
> > according to the use of exported symbols, like it does with modules.
>
> Yeah, that's indeed rather strange. There must be something in the amdkfd code
> which broke that somehow.
IMO, that's a far-fetched guess. Could you point to something more specific ?

> As far as I understand you the desired init order is radeon and amd_iommu_v2
> first and then amdkfd, right?
Actually no. The preferred order is amd_iommu_v2, amdkfd and radeon last. This 
is the order that happens when all three are built as modules. More accurately, 
radeon inits, but its init triggers amdkfd init, which triggers amd_iommu_v2 
init. So before radeon reaches its probe stage, all the modules were initialized.

So what happens when you boot with radeon,
> amd_iommu_v2 and amdkfd blacklisted for automatically load and only load amdkfd
> manually?
As said above, that's ok.
> > There will always be dependencies between kgd (radeon) and amdkfd and between
> > amdkfd and amd_iommu_v2. I don't think I can eliminate those dependencies, not
> > without a very complex solution. And the fact that this complex solution
> > occurs only in a very specific use case (all modules compiled in), makes me
> > less inclined to do that.
> >
> > So I don't see it as a "nasty workaround". I would call it just a "workaround"
> > for a specific use case, which should be solved by a generic solution to the
> > kernel enforcing load orders.
>
> The normal kernel module handling already should provide the correct init order,
> so I would still call this a rather nasty workaround because we couldn't find
> the underlying problem.
Well, the normal kernel module handling doesn't work when all modules are 
compiled in. I'm not a huge expert on this issue so I had Joerg Roedel help me 
analyze this (thanks Joerg) and he agreed that there is no enforcement of order 
in this case.

> Christian.
>
> >     Oded
> > > > ---
> > > >   drivers/gpu/drm/amd/amdkfd/kfd_module.c | 5 ++---
> > > >   1 file changed, 2 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_module.c
> > > > b/drivers/gpu/drm/amd/amdkfd/kfd_module.c
> > > > index 95d5af1..236562f 100644
> > > > --- a/drivers/gpu/drm/amd/amdkfd/kfd_module.c
> > > > +++ b/drivers/gpu/drm/amd/amdkfd/kfd_module.c
> > > > @@ -34,7 +34,7 @@
> > > >   #define KFD_DRIVER_MINOR    7
> > > >   #define KFD_DRIVER_PATCHLEVEL    0
> > > > -const struct kfd2kgd_calls *kfd2kgd;
> > > > +const struct kfd2kgd_calls *kfd2kgd = NULL;
> > > >   static const struct kgd2kfd_calls kgd2kfd = {
> > > >       .exit        = kgd2kfd_exit,
> > > >       .probe        = kgd2kfd_probe,
> > > > @@ -84,14 +84,13 @@ EXPORT_SYMBOL(kgd2kfd_init);
> > > >   void kgd2kfd_exit(void)
> > > >   {
> > > > +    kfd2kgd = NULL;
> > > >   }
> > > >   static int __init kfd_module_init(void)
> > > >   {
> > > >       int err;
> > > > -    kfd2kgd = NULL;
> > > > -
> > > >       /* Verify module parameters */
> > > >       if ((sched_policy < KFD_SCHED_POLICY_HWS) ||
> > > >           (sched_policy > KFD_SCHED_POLICY_NO_HWS)) {
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