[PATCH] drm/amdgpu: Add braces to initialize task_info subojects

[Richard Smith]

On Wed, Sep 12, 2018 at 10:38 AM Nick Desaulniers <ndesaulniers at google.com>
wrote:

> On Tue, Sep 11, 2018 at 5:26 PM Nathan Chancellor
> <natechancellor at gmail.com> wrote:
> >
> > Clang warns if there are missing braces around a subobject
> > initializer.
> >
> > drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c:1447:41: warning: suggest braces
> > around initialization of subobject [-Wmissing-braces]
> >                 struct amdgpu_task_info task_info = { 0 };
> >                                                       ^
> >                                                       {}
> > 1 warning generated.
> >
> > drivers/gpu/drm/amd/amdgpu/gmc_v9_0.c:262:41: warning: suggest braces
> > around initialization of subobject [-Wmissing-braces]
> >                 struct amdgpu_task_info task_info = { 0 };
> >                                                       ^
> >                                                       {}
> > 1 warning generated.
> >
> > Reported-by: Nick Desaulniers <ndesaulniers at google.com>
> > Signed-off-by: Nathan Chancellor <natechancellor at gmail.com>
> > ---
> >  drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c | 2 +-
> >  drivers/gpu/drm/amd/amdgpu/gmc_v9_0.c | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c
> b/drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c
> > index 9333109b210d..968cc1b8cdff 100644
> > --- a/drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c
> > +++ b/drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c
> > @@ -1444,7 +1444,7 @@ static int gmc_v8_0_process_interrupt(struct
> amdgpu_device *adev,
> >                 gmc_v8_0_set_fault_enable_default(adev, false);
> >
> >         if (printk_ratelimit()) {
> > -               struct amdgpu_task_info task_info = { 0 };
> > +               struct amdgpu_task_info task_info = { { 0 } };
>
> Hi Nathan,
> Thanks for this patch.  I discussed this syntax with our language
> lawyers.  Turns out, this is not quite correct, as you're now saying
> "initialize the first subobject to zero, but not the rest of the
> object."  -Wmissing-field-initializers would highlight this, but it's
> not part of -Wall.  It would be more correct to zero initialize the
> full struct, including all of its subobjects with `= {};`.
>

Sorry, I think I've caused some confusion here.

Elements with an omitted initializer get implicitly zero-initialized. In
C++, it's idiomatic to write `= {}` to perform aggregate
zero-initialization, but in C, that's invalid because at least one
initializer is syntactically required within the braces. As a result, `=
{0}` is an idiomatic way to perform zero-initialization of an aggregate in
C. Clang intends to suppress the -Wmissing-braces in that case; if the
warning is still being produced in a recent version of Clang, that's a bug.
However, the warning suppression was added between Clang 5 and Clang 6, so
it's very plausible that the compiler being used here is simply older than
the warning fix.

(Long story short: the change here seems fine, but should be unnecessary as
of Clang 6.)


> >                 amdgpu_vm_get_task_info(adev, entry->pasid, &task_info);
> >
> > diff --git a/drivers/gpu/drm/amd/amdgpu/gmc_v9_0.c
> b/drivers/gpu/drm/amd/amdgpu/gmc_v9_0.c
> > index 72f8018fa2a8..a781a5027212 100644
> > --- a/drivers/gpu/drm/amd/amdgpu/gmc_v9_0.c
> > +++ b/drivers/gpu/drm/amd/amdgpu/gmc_v9_0.c
> > @@ -259,7 +259,7 @@ static int gmc_v9_0_process_interrupt(struct
> amdgpu_device *adev,
> >         }
> >
> >         if (printk_ratelimit()) {
> > -               struct amdgpu_task_info task_info = { 0 };
> > +               struct amdgpu_task_info task_info = { { 0 } };
> >
> >                 amdgpu_vm_get_task_info(adev, entry->pasid, &task_info);
> >
> > --
> > 2.18.0
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers
>
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