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

[James Morse <james.morse@arm.com>]

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.


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

James