linux-kernel - Re: [PATCH v2 05/14] dt-bindings: arm: mediatek: document the pcie mirror node on MT7622

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Date:   Wed, 6 Apr 2022 13:01:06 +0200
From:   Felix Fietkau <nbd@....name>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        netdev@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 05/14] dt-bindings: arm: mediatek: document the pcie
 mirror node on MT7622


On 06.04.22 10:20, Krzysztof Kozlowski wrote:
> On 05/04/2022 21:57, Felix Fietkau wrote:
>> From: Lorenzo Bianconi <lorenzo@...nel.org>
>> 
>> This patch adds the pcie mirror document bindings for MT7622 SoC.
>> The feature is used for intercepting PCIe MMIO access for the WED core
>> Add related info in mediatek-net bindings.
>> 
>> Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
>> Signed-off-by: Felix Fietkau <nbd@....name>
>> ---
>>  .../mediatek/mediatek,mt7622-pcie-mirror.yaml | 42 +++++++++++++++++++
> 
> Eh, I wanted to ask to not put it inside arm/, but judging by your usage
> - you did not create drivers for both of these (WED and PCIe mirror).
> 
> You only need them to expose address spaces via syscon.
> 
> This actually looks hacky. Either WED and PCIe mirror are part of
> network driver, then add the address spaces via "reg". If they are not,
> but instead they are separate blocks, why you don't have drivers for them?
The code that uses the WED block is built into the Ethernet driver, but 
not all SoCs that use this ethernet core have it. Also, there are two 
WED blocks, and I'm not sure if future SoCs might have a different 
number of them at some point.
The WED code also needs to access registers of the ethernet MAC.
One reason for having a separate device is this:
As long as WED is not in use, ethernet supports coherent DMA for 
increased performance. When the first wireless device attaches to WED, 
IO coherency gets disabled and the ethernet DMA rings are cleaned up and 
allocated again, this time with the struct device of WED (which doesn't 
have the dma-coherent property).

- Felix