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SubjectRe: [PATCH v3] devres: Explicitly align datai[] to 64-bit
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Hi Peter,

On Mon, 2018-07-09 at 15:54 +0200, Peter Zijlstra wrote:
> On Mon, Jul 09, 2018 at 04:45:50PM +0300, Alexey Brodkin wrote:
> > diff --git a/drivers/base/devres.c b/drivers/base/devres.c
> > index f98a097e73f2..d65327cb83c9 100644
> > --- a/drivers/base/devres.c
> > +++ b/drivers/base/devres.c
> > @@ -24,8 +24,12 @@ struct devres_node {
> >
> > struct devres {
> > struct devres_node node;
> > - /* -- 3 pointers */
> > - unsigned long long data[]; /* guarantee ull alignment */
> > + /*
> > + * data[] must be 64 bit aligned even on 32 bit architectures
> > + * because it might be accessed by instructions that require
> > + * aligned memory arguments such as atomic64_t.
> > + */
> > + u8 __aligned(8) data[];
> > };
>
> From a quick reading in Documentation/driver-model/devres.txt this
> devres muck is supposed to be device memory, right?
>
> atomics (as in atomic*_t) are not defined for use on mmio.
>
> wth kind of code is relying on this?

It's Etnaviv (GPU/DRM) driver in etnaviv_gpu_platform_probe(),
see https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/etnaviv/etnaviv_gpu.c#L1740:
---------------------->8---------------------
struct drm_gpu_scheduler {
...
atomic64_t job_id_count;
...
};

struct etnaviv_gpu {
...
struct drm_gpu_scheduler sched;
};

struct etnaviv_gpu *gpu;

gpu = devm_kzalloc(dev, sizeof(*gpu), GFP_KERNEL);
---------------------->8---------------------

-Alexey
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