linux-kernel - Re: [PATCH v1] spi: fix client driver breakages when using GPIO descriptors

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Date:   Wed, 11 Nov 2020 02:05:19 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Sven Van Asbroeck <thesven73@...il.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Jonathan Cameron <jonathan.cameron@...wei.com>,
        Simon Han <z.han@...bus.com>, Lukas Wunner <lukas@...ner.de>,
        linux-spi <linux-spi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] spi: fix client driver breakages when using GPIO descriptors

On Mon, Nov 9, 2020 at 3:41 PM Sven Van Asbroeck <thesven73@...il.com> wrote:
> On Mon, Nov 9, 2020 at 9:24 AM Andy Shevchenko
> <andy.shevchenko@...il.com> wrote:
> >
> > Sounds like "many SPI drivers have to be fixed".
>
> I don't disagree. Fact is that after the imx cspi bus driver was converted
> to gpio descriptors, most spi client drivers broke. It would be great if this
> could be fixed. Any method that the community can find a consensus on,
> would be great :)

I think your patch is the quick fix.

I would say that anything that has:

spi->mode = ...

is essentially broken.

The core sets up vital things in .mode from e.g. device tree in
of_spi_parse_dt():

        /* Mode (clock phase/polarity/etc.) */
        if (of_property_read_bool(nc, "spi-cpha"))
                spi->mode |= SPI_CPHA;
        if (of_property_read_bool(nc, "spi-cpol"))
                spi->mode |= SPI_CPOL;
        if (of_property_read_bool(nc, "spi-3wire"))
                spi->mode |= SPI_3WIRE;
        if (of_property_read_bool(nc, "spi-lsb-first"))
                spi->mode |= SPI_LSB_FIRST;
        if (of_property_read_bool(nc, "spi-cs-high"))
                spi->mode |= SPI_CS_HIGH;

All this gets overwritten and ignored when a client just assigns mode
like that. Not just SPI_CS_HIGH. I doubt things are different
with ACPI.

> One the one hand: the fact that many spi client drivers just overwrite
> flags and values in their parent bus structure, doesn't sound idiomatic.
> I guess those spi->... values should really be opaque, and we should
> be using accessor functions, eg.:
>
>     static int acme_probe(struct spi_device *spi)
>     {
>         ...
>         // won't touch SPI_CS_HIGH flag
>         spi_set_mode_clock(spi, SPI_MODE_0);
>         ...
>     }

I would just make sure to affect the flags that matters to my driver,
it's just bits.

spi->mode &= ~FOO;
spi->mode |= BAR;

> On the other hand, it sounds very confusing to set SPI_CS_HIGH on
> all spi buses that use gpio descriptors: especially because gpiolib
> already handles absolutely everything related to polarity.

As long as gpiolib gets a 1 for asserted and a 0 for deasserted
it will be happy.

I'm not against your patch, it makes the codepath cleaner
so in a way it is good.

Yours,
Linus Walleij