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

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Message-ID: <CAMuHMdUv8FFwkde8K3Ta8FEWrkkJ=9ZqbTi1EO8sRxVOhGtvzQ@mail.gmail.com>
Date:   Wed, 27 Sep 2023 11:10:58 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Mark Brown <broonie@...nel.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

Hi Mark,

On Wed, Sep 27, 2023 at 11:00 AM Mark Brown <broonie@...nel.org> wrote:
> On Wed, Sep 27, 2023 at 09:59:05AM +0200, Geert Uytterhoeven wrote:
> > On Tue, Sep 26, 2023 at 11:08 PM Fabrizio Castro
> > > +    type: boolean
> > > +    description:
> > > +      Use CSI Slave Selection (SS) pin to enable transmission and reception when
> > > +      in slave mode.
>
> > Can't this be done in a more generic way? I had expected that the
> > existing SPI_NO_CS flag can be set using a property in the "slave" subnode,
> > but apparently there is no "spi-no-cs" property defined yet.
>
> 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".

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds