linux-kernel - Re: [PATCH 02/33] drivers: base: cacheinfo: Add helper to find the cache size from cpu+level

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Message-ID: <2ac66605-ee6a-495b-a0fb-910926abd8b0@arm.com>
Date: Wed, 27 Aug 2025 18:11:43 +0100
From: James Morse <james.morse@....com>
To: Dave Martin <Dave.Martin@....com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
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Subject: Re: [PATCH 02/33] drivers: base: cacheinfo: Add helper to find the
 cache size from cpu+level

Hi Dave,

On 27/08/2025 11:46, Dave Martin wrote:
> Hi,
> 
> On Fri, Aug 22, 2025 at 03:29:43PM +0000, James Morse wrote:
>> MPAM needs to know the size of a cache associated with a particular CPU.
>> The DT/ACPI agnostic way of doing this is to ask cacheinfo.
>>
>> Add a helper to do this.

>> diff --git a/include/linux/cacheinfo.h b/include/linux/cacheinfo.h
>> index 2dcbb69139e9..e12d6f2c6a57 100644
>> --- a/include/linux/cacheinfo.h
>> +++ b/include/linux/cacheinfo.h
>> @@ -148,6 +148,21 @@ static inline int get_cpu_cacheinfo_id(int cpu, int level)
>>  	return ci ? ci->id : -1;
>>  }
>>  
>> +/**
>> + * get_cpu_cacheinfo_size() - Get the size of the cache.
>> + * @cpu:      The cpu that is associated with the cache.
>> + * @level:    The level of the cache as seen by @cpu.
>> + *
>> + * Callers must hold the cpuhp lock.
>> + * Returns the cache-size on success, or 0 for an error.
>> + */
> 
> Nit: Maybe use the wording
> 
> 	cpuhp lock must be held.
> 
> in the kerneldoc here, to match the other helpers it sits alongside.
> 
> Otherwise, looks reasonable.

Sure,


>> +static inline unsigned int get_cpu_cacheinfo_size(int cpu, int level)
>> +{
>> +	struct cacheinfo *ci = get_cpu_cacheinfo_level(cpu, level);
>> +
>> +	return ci ? ci->size : 0;
>> +}
>> +
> 
> Orphaned function?
> 
> Can fs/resctrl/rdtgroup.c:rdtgroup_cbm_to_size() be ported to use this?
> If so, this wouldn't just be dead code in this series.

Ah - I thought the MPAM driver was pulling this value in, but its the resctrl glue code.
I was trying to reduce the number of trees this touches - its probably best to kick this
into the next series that adds the resctrl code as its pretty trivial.


Thanks,

James