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

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Date:   Wed, 27 Sep 2023 11:44:17 +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 11:21 AM Mark Brown <broonie@...nel.org> wrote:
> 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?

On RZ/V2M there isn't, as there is only a single hardware chip select.

On MSIOF, there are 3 hardware chip selects, but apparently only the
primary one can be used in target mode.

I have to admit I never thought about this before (commit
cf9e4784f3bde3e4 ("spi: sh-msiof: Add slave mode support") also predates
commit 9cce882bedd2768d ("spi: sh-msiof: Extend support to 3 native chip
selects")).  Hence the SPI target DT bindings use a single "slave" subnode,
without a unit address, thus assuming no explicit (or a default)
chip select configuration.

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