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

[Robin Murphy <robin.murphy@arm.com>]

On 11/04/2025 1:54 pm, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> In general a 'struct device' is way too large to be put on the kernel
> stack. Apparently something just caused it to grow a slightly larger,
> which pushed the arm_lpae_do_selftests() function over the warning
> limit in some configurations:
> 
> drivers/iommu/io-pgtable-arm.c:1423:19: error: stack frame size (1032) exceeds limit (1024) in 'arm_lpae_do_selftests' [-Werror,-Wframe-larger-than]
>   1423 | static int __init arm_lpae_do_selftests(void)
>        |                   ^
> 
> Change the function to use a dynamically allocated platform_device
> instead of the on-stack device structure. The device is not actually
> registered with the system, but is initialized enough to be used here.
> 
> Fixes: ca25ec247aad ("iommu/io-pgtable-arm: Remove iommu_dev==NULL special case")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>   drivers/iommu/io-pgtable-arm.c | 13 +++++++++----
>   1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
> index 7632c80edea6..9f3bf0b5e8da 100644
> --- a/drivers/iommu/io-pgtable-arm.c
> +++ b/drivers/iommu/io-pgtable-arm.c
> @@ -13,6 +13,7 @@
>   #include <linux/bitops.h>
>   #include <linux/io-pgtable.h>
>   #include <linux/kernel.h>
> +#include <linux/platform_device.h>
>   #include <linux/sizes.h>
>   #include <linux/slab.h>
>   #include <linux/types.h>
> @@ -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...)

> +	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.

Thanks,
Robin.

> +	if (!pdev)
> +		return -ENOMEM;
> +
> +	cfg.iommu_dev = &pdev->dev;
>   
>   	for (i = 0; i < ARRAY_SIZE(pgsize); ++i) {
>   		for (j = 0; j < ARRAY_SIZE(address_size); ++j) {
> @@ -1461,6 +1464,8 @@ static int __init arm_lpae_do_selftests(void)
>   	}
>   
>   	pr_info("selftest: completed with %d PASS %d FAIL\n", pass, fail);
> +	platform_device_put(pdev);
> +
>   	return fail ? -EFAULT : 0;
>   }
>   subsys_initcall(arm_lpae_do_selftests);