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

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Message-ID: <CAMuHMdXK92qO8KB6ejc6LLmfFsy=dZY18vNJGh+CKRZBAov-JA@mail.gmail.com>
Date:   Mon, 6 Jul 2020 21:57:53 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Adrian Fiergolski <adrian.fiergolski@...tree3d.com>
Cc:     Lukas Wunner <lukas@...ner.de>, Mark Brown <broonie@...nel.org>,
        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.

Hi Adrian,

On Mon, Jul 6, 2020 at 6:18 PM Mark Brown <broonie@...nel.org> wrote:
> On Mon, Jul 06, 2020 at 11:22:43AM +0200, Adrian Fiergolski wrote:
> > The implementation is transparent for the SPI devices and doesn't require
> > their modifications. It is based on a virtual SPI device (spi-daisy_chain)
> > and defines two required device tree properties ('spi-daisy-chain-len' and
> > 'spi-daisy-chain-noop') and one optional
>
> 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.

> Perhaps this needs to be a library for devices that can daisy
> chain?  It does feel like the instances should be aware of each other
> since half the point with building the hardware like this is that it
> enables operations on multiple devices to happen in sync.

Indeed. Exposing that functionality is the interesting, but hard part.

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