Re: [PATCH v3] devres: Explicitly align datai[] to 64-bit

[Alexey Brodkin <Alexey.Brodkin@synopsys.com>]

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