linux-kernel - Re: [PATCH v3 13/15] memory: mtk-smi: Get rid of need

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Date:   Mon, 3 Dec 2018 16:40:51 +0800
From:   Yong Wu <yong.wu@...iatek.com>
To:     Matthias Brugger <matthias.bgg@...il.com>
CC:     Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        "Rob Herring" <robh+dt@...nel.org>, <youlin.pei@...iatek.com>,
        <devicetree@...r.kernel.org>,
        Nicolas Boichat <drinkcat@...omium.org>, <arnd@...db.de>,
        <srv_heupstream@...iatek.com>, Will Deacon <will.deacon@....com>,
        <linux-kernel@...r.kernel.org>, Tomasz Figa <tfiga@...gle.com>,
        <iommu@...ts.linux-foundation.org>,
        <linux-mediatek@...ts.infradead.org>,
        Arvind Yadav <arvind.yadav.cs@...il.com>,
        <yingjoe.chen@...iatek.com>,
        <linux-arm-kernel@...ts.infradead.org>, <YT.shen@...iatek.com>
Subject: Re: [PATCH v3 13/15] memory: mtk-smi: Get rid of need_larbid

On Mon, 2018-12-03 at 00:04 +0100, Matthias Brugger wrote:
> 
> On 17/11/2018 03:35, Yong Wu wrote:
> > The "mediatek,larb-id" has already been parsed in MTK IOMMU driver.
> > It's no need to parse it again in SMI driver. Only clean some codes.
> > This patch is fit for all the current mt2701, mt2712, mt7623, mt8173
> > and mt8183.
> 
> I'm trying to understand why we need the mediatek,larb-id at all. From what I
> understand as long as the mediatek larbs described in the iommu are ordered
> (larb0, larb1, larb2, etc) we actually get the same value as defined by
> mediatek,larb-id. At least this holds for all present implementations.
> 
> On the other hand I don't see where the mtk_iommu_v1 driver actually parses the
> larb-id, can you please help to understand:
> 
> 1) why we need the larb-id at all

Actually only mt2712 which have 2 M4U HW need "mediatek,larb-id".

This is larbs in the m4u0/1 dtsi node of mt2712:
m4u0 { mediatek,larbs = <&larb0 &larb1 &larb2 &larb3 &larb6>;}
m4u1 { mediatek,larbs = <&larb4 &larb5 &larb7>;}

the id don't increase one by one, M4U have to get the larbid with the
help of "mediatek,larb-id".

(The m4u/smi dtsi patch of mt2712 will be send with some other modules,
maybe in this week.)


> 2) how this will work if we do not parse the larb-id for v1 iommu at all

As you said above and I also have wrote that the larbid "must sort
according to the local arbiter index" in the "mediatek,larbs"
description of dt-binding. All the M4U except mt2712 could ignore
"mediatek,larb-id". the v1 iommu also should be ok.

I'm not sure whether we should change [1], if only reserving mt2712
there, we also should change the dtsi file of mt2701 and mt7623.or keep
it as is.

[1]
https://elixir.bootlin.com/linux/v4.20-rc1/source/Documentation/devicetree/bindings/memory-controllers/mediatek,smi-larb.txt#L20

> 
> Thanks a lot,
> Matthias
> 
> > 

[snip]