Re: [PATCH] iommu/io-pgtable-arm: dynamically allocate selftest device struct

[Robin Murphy]

On 11/04/2025 2:44 pm, Arnd Bergmann wrote:
> On Fri, Apr 11, 2025, at 15:19, Robin Murphy wrote:
> > On 11/04/2025 1:54 pm, Arnd Bergmann wrote:
> > > @@ -1433,15 +1434,17 @@ static int __init arm_lpae_do_selftests(void)
> > >    	};
> > >    
> > >    	int i, j, k, pass = 0, fail = 0;
> > > -	struct device dev;
> >
> > Could we not simply make this static? Per the comment it's only here to
> > serve a NUMA node lookup buried deep in the pagetable allocator (TBH my
> > first thought was to just put an int on the stack and contrive a pointer
> > as the inverse of dev_to_node(), but I decided that would probably be
> > too contentious...)
>
> A static device would work here, but that has other (small)
> downsides:
>
>   - static devices are discouraged for any real purpose because
>     of the problematic lifetime rules. I think Greg would still
>     want to eliminate these entirely.
>
>   - there is slightly more memory usage: the __init function
>     gets eliminated after boot, while a static allocation says
>     around. It could perhaps be made __initdata.
>
>   - If we ever need anything beyond the NUMA node from it, the
>     dynamic allocation is probably close enough to make that
>     work.
>
> > > +	struct platform_device *pdev;
> > >    	struct io_pgtable_cfg cfg = {
> > >    		.tlb = &dummy_tlb_ops,
> > >    		.coherent_walk = true,
> > > -		.iommu_dev = &dev,
> > >    	};
> > >    
> > > -	/* __arm_lpae_alloc_pages() merely needs dev_to_node() to work */
> > > -	set_dev_node(&dev, NUMA_NO_NODE);
> > > +	pdev = platform_device_alloc("io-pgtable-test", 0);
> >
> > Otherwise, this would seem to be another perfect case for the new
> > faux_device.
>
> Good point, that is clearly better than platform_device in this
> case. Shall I send a new version with that?

Sure, I'm happy to consciously err on the side of caution and 
robustness, just making sure :)

Thanks,
Robin.