linux-kernel - Re: [PATCH] drm/amdkfd: fix build error with missing AMD_IOMMU

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Date:   Mon, 8 Mar 2021 21:35:54 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Christian König <ckoenig.leichtzumerken@...il.com>
Cc:     Felix Kuehling <felix.kuehling@....com>,
        Philip Yang <philip.yang@....com>,
        David Airlie <airlied@...ux.ie>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Alex Deucher <alexander.deucher@....com>,
        Colin Ian King <colin.king@...onical.com>,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH] drm/amdkfd: fix build error with missing AMD_IOMMU_V2

On Mon, Mar 8, 2021 at 9:12 PM Christian König
<ckoenig.leichtzumerken@...il.com> wrote:
> Am 08.03.21 um 21:02 schrieb Felix Kuehling:
> > Am 2021-03-08 um 2:33 p.m. schrieb Arnd Bergmann:

> > I don't want to create a hard dependency on AMD_IOMMU_V2 if I can avoid
> > it, because it is only really needed for a small number of AMD APUs, and
> > even there it is now optional for more recent ones.
> >
> > Is there a better way to avoid build failures without creating a hard
> > dependency?
>
> What you need is the same trick we used for AGP on radeon/nouveau:
>
> depends on AMD_IOMMU_V2 || !AMD_IOMMU_V2
>
> This way when AMD_IOMMU_V2 is build as a module DRM_AMDGPU will be build
> as a module as well. When it is disabled completely we don't care.

Note that this trick only works if you put it into the DRM_AMDGPU Kconfig option
itself, since that decides if the driver is built-in or a loadable module. If
HSA_AMD is disabled, that dependency is not really necessary.

The version I suggested  -- adding "depends on AMD_IOMMU_V2=y ||
DRM_AMDGPU=m" to the HSA_AMD option -- might be slightly nicer
since it lets you still build a kernel with DRM_AMDGPU=y and
AMD_IOMMU_V2=m, but without the HSA_AMD.

I can send a patch with either of those two options to replace my
original patch.

> >    The documentation in
> > Documentation/kbuild/kconfig-language.rst suggests using if
> > (IS_REACHABLE(CONFIG_AMD_IOMMU_V2)) to guard those problematic function
> > calls. I think more generally, we could guard all of kfd_iommu.c with
> >
> >      #if IS_REACHABLE(CONFIG_AMD_IOMMU_V2)
> >
> > And use the same condition to define the stubs in kfd_iommu.h.

This would fix the compile-time error, but it's also the one I'd least
recommend out of all the options, because that causes the driver to
silently not work as expected. This seems even worse than failing
the build.

       Arnd