linux-kernel - Re: [PATCH v2 3/6] dt-bindings: PCI: ti,am65: Extend for use with PVU

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Message-ID: <2bb6b337-d83b-4cbe-aff2-bbd0c7d17c2b@siemens.com>
Date: Tue, 27 Aug 2024 11:32:02 +0200
From: Jan Kiszka <jan.kiszka@...mens.com>
To: Krzysztof Kozlowski <krzk@...nel.org>
Cc: Nishanth Menon <nm@...com>, Santosh Shilimkar <ssantosh@...nel.org>,
 Vignesh Raghavendra <vigneshr@...com>, Rob Herring <robh+dt@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Conor Dooley <conor+dt@...nel.org>, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
 Siddharth Vadapalli <s-vadapalli@...com>,
 Bao Cheng Su <baocheng.su@...mens.com>, Hua Qian Li
 <huaqian.li@...mens.com>, Diogo Ivo <diogo.ivo@...mens.com>
Subject: Re: [PATCH v2 3/6] dt-bindings: PCI: ti,am65: Extend for use with PVU

On 27.08.24 11:29, Krzysztof Kozlowski wrote:
> On 27/08/2024 11:22, Jan Kiszka wrote:
>> On 27.08.24 08:37, Krzysztof Kozlowski wrote:
>>> On Mon, Aug 26, 2024 at 11:50:04PM +0200, Jan Kiszka wrote:
>>>> From: Jan Kiszka <jan.kiszka@...mens.com>
>>>>
>>>> Describe also the VMAP registers which are needed in order to make use
>>>> of the PVU with this PCI host. Furthermore, permit to specify a
>>>> restricted DMA pool by phandle.
>>>
>>> That's an ABI break without explanation why it is necessary.
>>>
>>
>> It is needed in order to support the PVU, as written above.
> 
> Above say only that you want a new feature and that's not really
> suitable explanation for ABI break, because answer to this is: add new
> feature without breaking existing users. But maybe there is a bug or
> something does not work or never work or there are no users, don't know.
> 
>>
>> Previous versions of this binding likely didn't consider this use case
>> and therefore didn't describe all registers associated with the hardware.
>>
>> BTW, if you see a way to add the required registers without breaking
>> more than needed, I'm all ears. At least the kernel driver will continue
>> to work with older DTs when you disable PVU support or do not add a DMA
>> pool to the DT.
> 
> If there is no ABI break, because driver still handles correctly old
> DTB, then mention it in the commit msg.

Well, this is strictly spoken not a topic for this commit because this
one should have no clue about what drivers do with DTs according to this
binding. But I can put a hint and go into details in the driver patch.

Jan

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