linux-kernel - Re: Two questions about cache coherency on arm platforms

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Date:   Tue, 24 Mar 2020 00:02:15 +0000
From:   Changbin Du <changbin.du@...il.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Changbin Du <changbin.du@...il.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: Two questions about cache coherency on arm platforms

On Mon, Mar 23, 2020 at 04:47:24PM +0000, Mark Rutland wrote:
> On Mon, Mar 23, 2020 at 04:15:40PM +0000, Changbin Du wrote:
> > Hi Mark,
> > Thanks for your answer. I still don't understand the first question.
> > 
> > On Mon, Mar 23, 2020 at 01:17:20PM +0000, Mark Rutland wrote:
> > > On Mon, Mar 23, 2020 at 08:35:26PM +0800, Changbin Du wrote:
> > > > Hi, All,
> > > > I am not very familiar with ARM processors. I have two questions about
> > > > cache coherency. Could anyone help me?
> > > > 
> > > > 1. How is cache coherency maintenanced on ARMv8 big.LITTLE system?
> > > > As far as I know, big cores and little cores are in seperate clusters on
> > > > big.LITTLE system.
> > > 
> > > This is often true, but not always the case. For example, with DSU big
> > > and little cores can be placed within the same cluster.
> > 
> > Yes, it is ture for DynamIQ that bl cores can be placed within the same cluster.
> > But I don't understand how linux support big.LITTLE before DynamIQ.
> 
> Multiple clusters can be in the same Inner Shareable domain, and Linux
> relies on this being the case for systems it supports. It's possible to
> build a system where clusters are in distinct Inner Shareable domains,
> but Linux does not support using all cores on such a system.
> 
> Even with CCI, CCN, CMN, etc, Linux requires that all cores (which it is
> told about) are in the same Inner Shareable domain. That is what is
> commonly built.
>
Thank you, I see now. I thought clusters must be in distinct Inner
Shareable domains. So I was wrong. The mannual is somewhat misleading.

> > I read below description in ARM Cortex-A Series Programmer’s Guide for
> > ARMv8-A.
> >  | big.LITTLE software models require transparent and efficient transfer of data between big and LITTLE clusters.
> >  | Coherency between clusters is provided by a cache-coherent interconnect such as the ARM CoreLink CCI-400 described in Chapter 14.
> > 
> > So I think  big cores and little cores are in different clusters in this
> > case. Then we are not within the same Inner Shareable domain?
> 
> Linux requires that those clusters are in the same Inner Shareable
> domain, and that's what people (mostly) build today.
> 
> Thanks,
> Mark.

-- 
Cheers,
Changbin Du