[RFC PATCH 04/36] cacheinfo: Expose the code to generate a cache-id from a device_node

[Ben Horgan]

Hi James,

On 7/25/25 18:08, James Morse wrote:
> Hi Ben,
> 
> On 14/07/2025 12:40, Ben Horgan wrote:
>> On 7/11/25 19:36, James Morse wrote:
>>> The MPAM driver identifies caches by id for use with resctrl. It
>>> needs to know the cache-id when probe-ing, but the value isn't set
>>> in cacheinfo until device_initcall().
>>>
>>> Expose the code that generates the cache-id. The parts of the MPAM
>>> driver that run early can use this to set up the resctrl structures
>>> before cacheinfo is ready in device_initcall().
> 
>>> diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
>>> index 613410705a47..0fdd6358ee73 100644
>>> --- a/drivers/base/cacheinfo.c
>>> +++ b/drivers/base/cacheinfo.c
>>> @@ -207,8 +207,7 @@ static bool match_cache_node(struct device_node *cpu,
>>>    #define arch_compact_of_hwid(_x)    (_x)
>>>    #endif
>>>    -static void cache_of_set_id(struct cacheinfo *this_leaf,
>>> -                struct device_node *cache_node)
>>> +unsigned long cache_of_calculate_id(struct device_node *cache_node)
>>>    {
>>>        struct device_node *cpu;
>>>        u32 min_id = ~0;
>>> @@ -219,15 +218,23 @@ static void cache_of_set_id(struct cacheinfo *this_leaf,
>>>            id = arch_compact_of_hwid(id);
>>>            if (FIELD_GET(GENMASK_ULL(63, 32), id)) {
>>>                of_node_put(cpu);
>>> -            return;
>>> +            return ~0UL;
>>>            }
>>>              if (match_cache_node(cpu, cache_node))
>>>                min_id = min(min_id, id);
>>>        }
>>>    -    if (min_id != ~0) {
>>> -        this_leaf->id = min_id;
>>> +    return min_id;
> 
>> Looks like some 32bit/64bit confusion. Don't we want to return ~0UL if min_id == ~0?
> 
> Certainly some confusion - yup, because cache_of_calculate_id() needs to return something
> that is out of range and (u32)-1 might be valid...
> 
> I think changing min_id to be defined as:
> |	unsigned long min_id = ~0UL;
> 
> fixes this - any trip round the loop that doesn't match anything will eventually return ~0UL.
Yes, that would work.

> 
> 
> Thanks! - I always get the 'UL' suffixes wrong.
> 
> James

Thanks,

Ben