linux-kernel - Re: [PATCH v2 1/2] spi: Add the SPI daisy chain support.

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Date:   Tue, 7 Jul 2020 11:25:11 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Adrian Fiergolski <adrian.fiergolski@...tree3d.com>,
        Lukas Wunner <lukas@...ner.de>,
        kernel test robot <lkp@...el.com>,
        Rob Herring <robh+dt@...nel.org>,
        linux-spi <linux-spi@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] spi: Add the SPI daisy chain support.

On Mon, Jul 06, 2020 at 09:57:53PM +0200, Geert Uytterhoeven wrote:
> On Mon, Jul 6, 2020 at 6:18 PM Mark Brown <broonie@...nel.org> wrote:

> > It would really help to have an example of how a client device will use
> > this, right now it's a bit hard to follow.  Overall it feels like this
> > should be better abstracted, right now there's lots of ifdefs throughout
> > the code which make things unclear and also seem like they're going to
> > be fragile long term since realistically very few systems will be using
> > this.

> Can't the ifdefs be avoided by implementing this as a new SPI controller?
> I.e. the daisy chain driver will operate as a slave of the parent SPI
> controller,
> but will expose a new SPI bus to the daisy-chained slaves.

Yes, that might work.  I do worry about locking issues with having a SPI
controller connected via SPI but we mostly only lock at the controller
level so it's probably fine.  Not sure how this would perform either.

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