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: <aLBjTyTKk6Pod40v@e133380.arm.com>
Date: Thu, 28 Aug 2025 15:10:23 +0100
From: Dave Martin <Dave.Martin@....com>
To: James Morse <james.morse@....com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-acpi@...r.kernel.org, devicetree@...r.kernel.org,
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	Danilo Krummrich <dakr@...nel.org>
Subject: Re: [PATCH 02/33] drivers: base: cacheinfo: Add helper to find the
 cache size from cpu+level

Hi James,

On Wed, Aug 27, 2025 at 06:11:43PM +0100, James Morse wrote:
> 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

Sure, that also works.

Cheers
---Dave