linux-kernel - Re: [PATCH 16/21] dt-bindings: reserved-memory: introduce designated-movable-block

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Message-ID: <e0e043aa-0f79-59a8-05ab-e48046860524@linaro.org>
Date:   Mon, 19 Sep 2022 13:03:46 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Doug Berger <opendmb@...il.com>, Rob Herring <robh@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Frank Rowand <frowand.list@...il.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Mike Rapoport <rppt@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Borislav Petkov <bp@...e.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        David Hildenbrand <david@...hat.com>, Zi Yan <ziy@...dia.com>,
        Oscar Salvador <osalvador@...e.de>,
        Hari Bathini <hbathini@...ux.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        - <devicetree-spec@...r.kernel.org>,
        KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
        Mel Gorman <mgorman@...e.de>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux.dev
Subject: Re: [PATCH 16/21] dt-bindings: reserved-memory: introduce
 designated-movable-block

On 19/09/2022 01:12, Doug Berger wrote:
> On 9/18/2022 3:31 AM, Krzysztof Kozlowski wrote:
>> On 14/09/2022 18:13, Doug Berger wrote:
>>> On 9/14/2022 7:55 AM, Rob Herring wrote:
>>>> On Tue, Sep 13, 2022 at 12:55:03PM -0700, Doug Berger wrote:
>>>>> Introduce designated-movable-block.yaml to document the
>>>>> devicetree binding for Designated Movable Block children of the
>>>>> reserved-memory node.
>>>>
>>>> What is a Designated Movable Block? This patch needs to stand on its
>>>> own.
>>> As noted in my reply to your [PATCH 00/21] comment, my intention in
>>> submitting the entire patch set (and specifically PATCH 00/21]) was to
>>> communicate this context. Now that I believe I understand that only this
>>> patch should have been submitted to the devicetree-spec mailing list, I
>>> will strive harder to make it more self contained.
>>
>> The submission of entire thread was ok. What is missing is the
>> explanation in this commit. This commit must be self-explanatory (e.g.
>> in explaining "Why are you doing it?"), not rely on other commits for
>> such explanation.
>>
>>>
>>>>
>>>> Why does this belong or need to be in DT?
>>> While my preferred method of declaring Designated Movable Blocks is
>>> through the movablecore kernel parameter, I can conceive that others may
>>> wish to take advantage of the reserved-memory DT nodes. In particular,
>>> it has the advantage that a device can claim ownership of the
>>> reserved-memory via device tree, which is something that has yet to be
>>> implemented for DMBs defined with movablecore.
>>
>> Rephrasing the question: why OS memory layout and OS behavior is a
>> property of hardware (DTS)?
> I would say the premise is fundamentally the same as the existing 
> reserved-memory child node.

I don't think it is fundamentally the same.

The existing reserved-memory node describes memory used by hardware - by
other devices. The OS way of handling this memory - movable, reclaimable
etc - is not part of it.

So no, it is not the same.

> 
> I've been rethinking how this should be specified. I am now thinking 
> that it may be better to introduce a new Reserved Memory property that 
> serves as a modifier to the 'reusable' property. The 'reusable' property 
> allows the OS to use memory that has been reserved for a device and 
> therefore requires the device driver to reclaim the memory prior to its 
> use. However, an OS may have multiple ways of implementing such reuse 
> and reclamation.

... and I repeat the question - why OS way of implementing reuse and
reclamation is relevant to DT?

> I am considering introducing the vendor specific 'linux,dmb' property 
> that is dependent on the 'reusable' property to allow both the OS and 
> the device driver to identify the method used by the Linux OS to support 
> reuse and reclamation of the reserved-memory child node.

Sure, but why? Why OS and Linux driver specific pieces should be in DT?
> Such a property would remove any need for new compatible strings to the 
> device tree. Does that approach seem reasonable to you?

No, because you did not explain original question. At all.

Best regards,
Krzysztof