linux-kernel - Re: [PATCH] ACPI/PPTT: Handle architecturally unknown cache types

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Date:   Wed, 12 Sep 2018 10:06:49 -0600
From:   Jeffrey Hugo <jhugo@...eaurora.org>
To:     Jeremy Linton <jeremy.linton@....com>, rjw@...ysocki.net,
        linux-acpi@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, vkilari@...eaurora.org,
        Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH] ACPI/PPTT: Handle architecturally unknown cache types

On 9/12/2018 9:39 AM, Jeremy Linton wrote:
> Hi,
> 
> 
> On 09/12/2018 09:41 AM, Jeffrey Hugo wrote:
>> The HW designers have indicated that there is no sane way to provide 
>> sets/ways information to software, even on an informational basis (ie 
>> not for cache maintenance, but for performance optimizations). 
>> Therefore the firmware will not provide this information because it 
>> will be wrong.
>>
>> So, therefore, we should still be able to tell the user that a cache 
>> exists at the relevant level, and what size it is.  On the concerned 
>> system, we cannot do that currently.
> 
> Ok, so set the fields to zero in firmware node, and mark them valid.

Is that what the PPTT spec says to do?

> That logically says that there isn't any set/way information for the 
> cache (which implies direct mapped).

Making inferences such as that have gotten folks into trouble when 
interpreting other specs.  Unfortunately I am not allowed to detail more 
about this specific system, however implying that the affected cache(s) 
are direct mapped is incorrect.  Officially, what you have is a cache or 
multiple caches at certain levels that have a specified size.  You can 
make no inferences about the exact behavior or implementation of the 
cache beyond what FW explicitly provides.  I'm not particularly a fan of 
it, but its what I have to deal with.

-- 
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.