linux-kernel - Re: [PATCH v3 2/4] cacheinfo: Check cache properties are present in DT

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Date:   Fri, 14 Apr 2023 10:05:42 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Pierre Gondois <pierre.gondois@....com>,
        linux-kernel@...r.kernel.org, Radu Rendec <rrendec@...hat.com>,
        Sudeep Holla <sudeep.holla@....com>,
        Alexandre Ghiti <alexghiti@...osinc.com>,
        Conor Dooley <conor.dooley@...rochip.com>,
        Will Deacon <will@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Palmer Dabbelt <palmer@...osinc.com>,
        Gavin Shan <gshan@...hat.com>
Subject: Re: [PATCH v3 2/4] cacheinfo: Check cache properties are present in
 DT

On Thu, Apr 13, 2023 at 01:06:46PM -0700, Florian Fainelli wrote:
> On 4/13/23 12:50, Sudeep Holla wrote:
> > On Thu, Apr 13, 2023 at 11:16:37AM -0700, Florian Fainelli wrote:
> > > On 4/13/23 02:14, Pierre Gondois wrote:
> > > > If a Device Tree (DT) is used, the presence of cache properties is
> > > > assumed. Not finding any is not considered. For arm64 platforms,
> > > > cache information can be fetched from the clidr_el1 register.
> > > > Checking whether cache information is available in the DT
> > > > allows to switch to using clidr_el1.
> > > > 
> > > > init_of_cache_level()
> > > > \-of_count_cache_leaves()
> > > > will assume there a 2 cache leaves (L1 data/instruction caches), which
> > > > can be different from clidr_el1 information.
> > > > 
> > > > cache_setup_of_node() tries to read cache properties in the DT.
> > > > If there are none, this is considered a success. Knowing no
> > > > information was available would allow to switch to using clidr_el1.
> > > > 
> > > > Fixes: de0df442ee49 ("cacheinfo: Check 'cache-unified' property to count cache leaves")
> > > > Reported-by: Alexandre Ghiti <alexghiti@...osinc.com>
> > > > Link: https://lore.kernel.org/all/20230404-hatred-swimmer-6fecdf33b57a@spud/
> > > > Signed-off-by: Pierre Gondois <pierre.gondois@....com>
> > > 
> > > Humm, it would appear that the cache levels and topology is still provided,
> > > despite the lack of cache properties in the Device Tree which is intended by
> > > this patch set however we lost the size/ways/sets information, could we not
> > > complement the missing properties here?
> > > 
> > 
> > I am confused. How and from where the information was fetched before this
> > change ?
> 
> I applied Pierre's patches to my tree and then did the following:
> 
> - before means booting with the patches applied and the Device Tree
> providing cache information: {d,i}-cache-{size,line-size,sets} and
> next-level-cache
> 
> - after means removing all of those properties still with the patches
> applied
>

Ah okay, I assumed something totally different and hence thought patches
broke something.

> My expectation is that if we omit the properties in the Device Tree, we will
> fallback to reading that information out of clidr_el1. However as can be
> seen from the "before" and "after" outputs, there is loss of information, as
> we no longer have the cacheline size, number of sets/ways, the rest is valid
> though.
>

Correct and that is expected. We dropped ccsidr_el1 support to fetch cache
geometry with the commit a8d4636f96ad ("arm64: cacheinfo: Remove CCSIDR-based
cache information probing") after Arm ARM added wordings not to infer the
information. However clidr_el1 info still holds except it may not include
transparent system level caches. Hope that clarifies.

> So my question is whether this is expected and in scope of what is being
> done here, or not.
> 
> > 
> > > If this is out of the scope of what you are doing:
> > > 
> > > Tested-by: Florian Fainelli <f.fainelli@...il.com>
> > > 
> > 
> > Just looking at the lscpu output before and after, it looks something is
> > broken. What am I missing here ?
> > 
> 
> What is broken in the "before" output? It contains the entire set of
> possible information we know about the caches. As for the "after", well yes
> there is information missing, the whole point of my email actually...

Sorry, I was not referring to "before" output. I assumed "before" means
without patches and "after" means with patches, hence I thought patches
broke something but got confused why are you giving tested-by :D.

-- 
Regards,
Sudeep