linux-kernel - Re: [PATCH 2/2] arm64: dts: imx8qxp: Add EDMA0/EDMA1 nodes

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Date:   Wed, 27 Mar 2019 18:36:30 +0200
From:   Daniel Baluta <daniel.baluta@...il.com>
To:     Lucas Stach <l.stach@...gutronix.de>
Cc:     Aisheng Dong <aisheng.dong@....com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Peng Fan <peng.fan@....com>,
        "festevam@...il.com" <festevam@...il.com>,
        Anson Huang <anson.huang@....com>, Teo Hall <teo.hall@....com>,
        Daniel Baluta <daniel.baluta@....com>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        dl-linux-imx <linux-imx@....com>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "S.j. Wang" <shengjiu.wang@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/2] arm64: dts: imx8qxp: Add EDMA0/EDMA1 nodes

On Wed, Mar 27, 2019 at 11:33 AM Lucas Stach <l.stach@...gutronix.de> wrote:
>
> Hi Daniel,
>
> Am Mittwoch, den 27.03.2019, 10:51 +0200 schrieb Daniel Baluta:
> [...]
> >
> > > or
> > > "fsl,imx8qxp-edma", "fsl,imx8qm-edma"?
> >
> > One thing that it is not clear for me is why there are places
> > where we use two compatible strings?
> >
> > I understand the situation where are two distinct drivers, but is there
> > any other reason to add multiple compatible strings for a node in dts?
>
> We use 2 compatible string where there should not be any differences
> between the IP blocks of this SoC and a version the driver already
> supports.
>
> So if the eDMA driver already supports the software interface for
> "fsl,imx8qm-edma" and the IP block is compatible with this, we add this
> to the DT, so the we don't need any driver changes just to support a
> new SoC. But as you can never be sure if there are subtle differences
> in the IP block and/or SOC integration when adding the DT support, we
> also add a more specific compatible to the DT. If it turns out that
> there are software visible differences, we only need to adapt the
> driver to check for the more specific compatible to trigger the changed
> behavior, allowing to keep the DT stable.

Excellent explanation Lucas! For the moment upstream eDMA driver doesn't have
support for any i.MX8 so I think using "fsl,imx8qxp-edma" should be acceptable.

Thanks,
Daniel.