linux-kernel - Re: [PATCH 1/8] gpio: Add Elba SoC gpio driver for spi cs control

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Date:   Thu, 4 Mar 2021 14:38:01 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Serge Semin <fancer.lancer@...il.com>
Cc:     Brad Larson <brad@...sando.io>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Mark Brown <broonie@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Olof Johansson <olof@...om.net>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-spi <linux-spi@...r.kernel.org>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/8] gpio: Add Elba SoC gpio driver for spi cs control

On Thu, Mar 4, 2021 at 10:10 AM Serge Semin <fancer.lancer@...il.com> wrote:
> On Thu, Mar 04, 2021 at 09:29:33AM +0100, Linus Walleij wrote:

> > > + * pin:             3            2        |       1            0
> > > + * bit:         7------6------5------4----|---3------2------1------0
> > > + *             cs1  cs1_ovr  cs0  cs0_ovr |  cs1  cs1_ovr  cs0  cs0_ovr
> > > + *                        ssi1            |             ssi0
> > > + */
> > > +#define SPICS_PIN_SHIFT(pin)   (2 * (pin))
> > > +#define SPICS_MASK(pin)                (0x3 << SPICS_PIN_SHIFT(pin))
> > > +#define SPICS_SET(pin, val)    ((((val) << 1) | 0x1) << SPICS_PIN_SHIFT(pin))
> >
>
> > So 2 bits per GPIO line in one register? (Nice doc!)
>
> I suppose the first bit is the CS-pin-override flag. So when it's set
> the output is directly driven by the second bit, otherwise the
> corresponding DW APB SPI controller drives it. That's how the
> multiplexing is implemented here.

If these output lines are so tightly coupled to the SPI block
and will not be used for any other GPO (general purpose output)
I think it makes more sense to bundle the handling into the
DW SPI driver, and activate it based on the Elba compatible
string (if of_is_compatible(...)).

I am a bit cautious because it has happened in the past that
people repurpose CS lines who were originally for SPI CS
to all kind of other purposes, such as a power-on LED and
in that case it needs to be a separate GPIO driver. So the
author needs to have a good idea about what is a realistic
use case here.

Yours,
Linus Walleij