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

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Message-ID: <0ca0fc20-421e-4bbc-bcda-9e5c2ea3b1b4@kernel.org>
Date: Tue, 27 Aug 2024 11:29:15 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Jan Kiszka <jan.kiszka@...mens.com>
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 27/08/2024 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.

Best regards,
Krzysztof