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

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Date:   Thu, 7 Apr 2022 12:16:55 -0500
From:   Rob Herring <robh@...nel.org>
To:     Felix Fietkau <nbd@....name>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        netdev@...r.kernel.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>,
        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 Wed, Apr 06, 2022 at 01:01:06PM +0200, Felix Fietkau wrote:
> 
> 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).

I'm pretty sure there are assumptions in the driver core that coherency 
is not changing on the fly. In any case, if it is, using 'dma-coherent' 
is not appropriate. You obviously have another method to determine 
whether you are coherent or not.

Rob