Re: [PATCH 2/5] cacheinfo: Add arch hook to compress CPU h/w id into 32 bits for cache-id

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Fri, 13 Jun 2025 13:03:53 +0000
James Morse <james.morse@arm.com> wrote:

> Filesystems like resctrl use the cache-id exposed via sysfs to identify
> groups of CPUs. The value is also used for PCIe cache steering tags. On
> DT platforms cache-id is not something that is described in the
> device-tree, but instead generated from the smallest CPU h/w id of the
> CPUs associated with that cache.
> 
> CPU h/w ids may be larger than 32 bits.
> 
> Add a hook to allow architectures to compress the value from the devicetree
> into 32 bits. Returning the same value is always safe as cache_of_set_id()
> will stop if a value larger than 32 bits is seen.
> 
> For example, on arm64 the value is the MPIDR affinity register, which only
> has 32 bits of affinity data, but spread across the 64 bit field. An
> arch-specific bit swizzle gives a 32 bit value.
> 
> Signed-off-by: James Morse <james.morse@arm.com>

Looks fine to me
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>

> ---
>  drivers/base/cacheinfo.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
> index 9888d87840a2..d8e5b4c7156c 100644
> --- a/drivers/base/cacheinfo.c
> +++ b/drivers/base/cacheinfo.c
> @@ -184,6 +184,10 @@ static bool cache_node_is_unified(struct cacheinfo *this_leaf,
>  	return of_property_read_bool(np, "cache-unified");
>  }
>  
> +#ifndef arch_compact_of_hwid
> +#define arch_compact_of_hwid(_x)	(_x)
> +#endif
> +
>  static void cache_of_set_id(struct cacheinfo *this_leaf, struct device_node *np)
>  {
>  	struct device_node *cpu;
> @@ -193,6 +197,7 @@ static void cache_of_set_id(struct cacheinfo *this_leaf, struct device_node *np)
>  		struct device_node *cache_node __free(device_node) = of_find_next_cache_node(cpu);
>  		u64 id = of_get_cpu_hwid(cpu, 0);
>  
> +		id = arch_compact_of_hwid(id);
>  		if (FIELD_GET(GENMASK_ULL(63, 32), id)) {
>  			of_node_put(cpu);
>  			return;