linux-kernel - Re: [PATCH v2 3/8] dt-bindings: media: mediatek,jpeg: Relax IOMMU max item count

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Message-ID: <9b4ee771-2772-46f7-b6f2-382c11934d99@kernel.org>
Date: Wed, 2 Oct 2024 16:00:43 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Matthias Brugger <matthias.bgg@...il.com>, Fei Shao <fshao@...omium.org>
Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
 Bin Liu <bin.liu@...iatek.com>, Conor Dooley <conor+dt@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Mauro Carvalho Chehab <mchehab@...nel.org>, Rob Herring <robh@...nel.org>,
 Xia Jiang <xia.jiang@...iatek.com>, devicetree@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-media@...r.kernel.org, linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH v2 3/8] dt-bindings: media: mediatek,jpeg: Relax IOMMU max
 item count

On 02/10/2024 15:12, Matthias Brugger wrote:
> 
> 
> On 02/10/2024 08:16, Krzysztof Kozlowski wrote:
>> On Tue, Oct 01, 2024 at 07:27:21PM +0800, Fei Shao wrote:
>>> On MediaTek platforms with Multimedia MMU (M4U), a multimedia hardware
>>> can be assigned with a local arbiter (LARB) which has a maximum of 32
>>> ports for MediaTek's IOMMU infrastructure. That means there can be at
>>> most 32 items in the iommus property in theory.
>>>
>>> Instead of relaxing the max item count every time a newly introduced
>>> device tree hits the limit, bump the number to 32 as an one-time effort.
>>>
>>> On the other hand, all existing and foreseeable JPEG decoder nodes at
>>> this point have at least 2 IOMMUs, so set minItems to 2 accordingly.
>>>
>>> Signed-off-by: Fei Shao <fshao@...omium.org>
>>> ---
>>> It's not in the upstream tree yet, but the upcoming MT8188 DT will have
>>> 6 IOMMUs in its JPEG decoder. This patch is to pave the way for that.
>>>
>>> Changes in v2:
>>> New patch.
>>>
>>
>> I do not see any user of this in this patchset.
>>
> 
> I'm not sure I understand you: the binding describes the hardware. If it can 
> have up to 32 ports, then this should be reflected in the binding.

Yeah, but people do not describe hardware but drivers. Looking at DTS
clarifies sometimes such cases and avoids lengthy discussions. Plus I
was not really sure if these are IOMMUs for the same device for example
(again: people do weird stuff).

Best regards,
Krzysztof