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

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Date:   Mon, 29 Mar 2021 19:44:48 -0700
From:   Brad Larson <brad@...sando.io>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Mark Brown <broonie@...nel.org>,
        Serge Semin <fancer.lancer@...il.com>,
        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 12:29 AM Linus Walleij <linus.walleij@...aro.org> wrote:
>
> Hi Brad,
>
> thanks for your patch!
>
> On Thu, Mar 4, 2021 at 4:42 AM Brad Larson <brad@...sando.io> wrote:
>
> > This GPIO driver is for the Pensando Elba SoC which
> > provides control of four chip selects on two SPI busses.
> >
> > Signed-off-by: Brad Larson <brad@...sando.io>
> (...)
>
> > +#include <linux/gpio.h>
>
> Use this in new drivers:
> #include <linux/gpio/driver.h>
>
> > + * 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!)
>
> > +struct elba_spics_priv {
> > +       void __iomem *base;
> > +       spinlock_t lock;
> > +       struct gpio_chip chip;
> > +};
> > +
> > +static int elba_spics_get_value(struct gpio_chip *chip, unsigned int pin)
> > +{
> > +       return -ENXIO;
> > +}
>
> Write a comment that the chip only supports output mode,
> because it repurposes SPI CS pins as generic GPIO out,
> maybe at the top of the file?
>

I'll add a comment regarding gpio pin mode.  Yes output
only mode as SPI chip-selects.

> I suppose these systems also actually (ab)use the SPI cs
> for things that are not really SPI CS? Because otherwise
> this could just be part of the SPI driver (native chip select).
>
> > +static const struct of_device_id ebla_spics_of_match[] = {
> > +       { .compatible = "pensando,elba-spics" },
>
> Have you documented this?

Yes in Documentation/devicetree/bindings, I'll double check
the content for completeness.  The SPI CS isn't used for
something else, the integrated DesignWare IP doesn't
support 4 chip-selects on two spi busses.

>
> Other than that this is a nice and complete driver.
>
> Yours,
> Linus Walleij

Thanks for the review!