linux-kernel - Re: [PATCH 1/2] spi: renesas,rzv2m-csi: Add SPI Slave related properties

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Message-ID: <ZRP0MpIHf67tfQJY@finisterre.sirena.org.uk>
Date:   Wed, 27 Sep 2023 11:21:54 +0200
From:   Mark Brown <broonie@...nel.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Fabrizio Castro <fabrizio.castro.jz@...esas.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Magnus Damm <magnus.damm@...il.com>, linux-spi@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org,
        Chris Paterson <Chris.Paterson2@...esas.com>,
        Biju Das <biju.das@...renesas.com>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: Re: [PATCH 1/2] spi: renesas,rzv2m-csi: Add SPI Slave related
 properties

On Wed, Sep 27, 2023 at 11:10:58AM +0200, Geert Uytterhoeven wrote:
> On Wed, Sep 27, 2023 at 11:00 AM Mark Brown <broonie@...nel.org> wrote:

> > The description is clearly saying there is a chip select, _NO_CS seems
> > entirely inappropriate.  It's not specified in the device tree because
> > when there's no chip select for a device it's a fundamental property of
> > how the device is controlled and we don't need any information beyond
> > the compatible.

> In host mode, it indeed doesn't matter, as you can have only a single
> device connected with SPI_NO_CS.
> In device mode, the device needs to know if it must monitor the chip
> select line or not.

> In hindsight, I should have kept the question I had written initially,
> but deleted after having read the documentation for the corresponding
> RZ/V2M register bits:

>     What does it mean if this is false? That there is no chip select?

> So "spi-no-cs" would be the inverse of "renesas,csi-ss".

I see.  Is there any control over what the chip select is when there is
one, in which case we could just look to see if there's one specified?

I'm a bit nervous about a generic property that maps onto _NO_CS since
it's likely that people will start using that in device bindings when
they shouldn't.

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