linux-kernel - Re: [PATCH v6 08/30] dt-bindings: soc: fsl: cpm_qe: cpm1-scc-qmc: Add support for QMC HDLC

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Message-ID: <20230925122758.43963736@bootlin.com>
Date:   Mon, 25 Sep 2023 12:27:58 +0200
From:   Herve Codina <herve.codina@...tlin.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Andrew Lunn <andrew@...n.ch>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>, Lee Jones <lee@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Qiang Zhao <qiang.zhao@....com>, Li Yang <leoyang.li@....com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        Shengjiu Wang <shengjiu.wang@...il.com>,
        Xiubo Li <Xiubo.Lee@...il.com>,
        Fabio Estevam <festevam@...il.com>,
        Nicolin Chen <nicoleotsuka@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Randy Dunlap <rdunlap@...radead.org>, netdev@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, alsa-devel@...a-project.org,
        Simon Horman <horms@...nel.org>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v6 08/30] dt-bindings: soc: fsl: cpm_qe: cpm1-scc-qmc:
 Add support for QMC HDLC

On Mon, 25 Sep 2023 10:21:15 +0200
Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org> wrote:

> On 25/09/2023 10:17, Herve Codina wrote:
> > Hi Krzysztof,
> > 
> > On Sat, 23 Sep 2023 19:39:49 +0200
> > Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org> wrote:
> >   
> >> On 22/09/2023 09:58, Herve Codina wrote:  
> >>> The QMC (QUICC mutichannel controller) is a controller present in some
> >>> PowerQUICC SoC such as MPC885.
> >>> The QMC HDLC uses the QMC controller to transfer HDLC data.
> >>>
> >>> Additionally, a framer can be connected to the QMC HDLC.
> >>> If present, this framer is the interface between the TDM bus used by the
> >>> QMC HDLC and the E1/T1 line.
> >>> The QMC HDLC can use this framer to get information about the E1/T1 line
> >>> and configure the E1/T1 line.
> >>>
> >>> Signed-off-by: Herve Codina <herve.codina@...tlin.com>
> >>> ---
> >>>  .../soc/fsl/cpm_qe/fsl,cpm1-scc-qmc.yaml      | 24 +++++++++++++++++++
> >>>  1 file changed, 24 insertions(+)
> >>>
> >>> diff --git a/Documentation/devicetree/bindings/soc/fsl/cpm_qe/fsl,cpm1-scc-qmc.yaml b/Documentation/devicetree/bindings/soc/fsl/cpm_qe/fsl,cpm1-scc-qmc.yaml
> >>> index 82d9beb48e00..61dfd5ef7407 100644
> >>> --- a/Documentation/devicetree/bindings/soc/fsl/cpm_qe/fsl,cpm1-scc-qmc.yaml
> >>> +++ b/Documentation/devicetree/bindings/soc/fsl/cpm_qe/fsl,cpm1-scc-qmc.yaml
> >>> @@ -101,6 +101,27 @@ patternProperties:
> >>>            Channel assigned Rx time-slots within the Rx time-slots routed by the
> >>>            TSA to this cell.
> >>>  
> >>> +      compatible:
> >>> +        const: fsl,qmc-hdlc    
> >>
> >> Why this is not a device/SoC specific compatible?  
> > 
> > This compatible is present in a QMC channel.
> > The parent node (the QMC itself) contains a compatible with device/SoC:
> > --- 8< ---
> >   compatible:
> >     items:
> >       - enum:
> >           - fsl,mpc885-scc-qmc
> >           - fsl,mpc866-scc-qmc
> >       - const: fsl,cpm1-scc-qmc
> > --- 8< ---
> > 
> > At the child level (ie QMC channel), I am not sure that adding device/SoC
> > makes sense. This compatible indicates that the QMC channel is handled by
> > the QMC HDLC driver.
> > At this level, whatever the device/SoC, we have to be QMC compliant.
> > 
> > With these details, do you still think I need to change the child (channel)
> > compatible ?  
> 
> From OS point of view, you have a driver binding to this child-level
> compatible. How do you enforce Linux driver binding based on parent
> compatible? I looked at your next patch and I did not see it.

We do not need to have the child driver binding based on parent.
We have to ensure that the child handles a QMC channel and the parent provides
a QMC channel.

A QMC controller (parent) has to implement the QMC API (include/soc/fsl/qe/qmc.h)
and a QMC channel driver (child) has to use the QMC API.

Best regards,
Hervé

> 
> Best regards,
> Krzysztof
>