linux-kernel - Re: [PATCH] spi: docs: improve the SPI userspace API documentation

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Message-ID: <CAMuHMdVcsfE6TZbu8SJZP7CNKyjwBZdBiN0nDRQCibaGgpLF0g@mail.gmail.com>
Date:   Fri, 19 Nov 2021 09:10:01 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Javier Martinez Canillas <javierm@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        linux-spi <linux-spi@...r.kernel.org>,
        Ralph Siemsen <ralph.siemsen@...aro.org>
Subject: Re: [PATCH] spi: docs: improve the SPI userspace API documentation

Hi Javier,

On Thu, Nov 18, 2021 at 10:32 PM Javier Martinez Canillas
<javierm@...hat.com> wrote:
> This doc is fairly outdated and only uses legacy device instantiation
> terminology. Let us update it and also mention the OF and ACPI device
> tables, to make easier for users to figure out how should be defined.
>
> Also, mention that devices bind could be done in user-space now using
> the "driver_override" sysfs entry.
>
> Suggested-by: Ralph Siemsen <ralph.siemsen@...aro.org>
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>

Thanks for your patch!

> --- a/Documentation/spi/spidev.rst
> +++ b/Documentation/spi/spidev.rst
> @@ -29,15 +29,39 @@ of the driver stack) that are not accessible to userspace.
>
>  DEVICE CREATION, DRIVER BINDING
>  ===============================
> -The simplest way to arrange to use this driver is to just list it in the
> -spi_board_info for a device as the driver it should use:  the "modalias"
> -entry is "spidev", matching the name of the driver exposing this API.
> +
> +The spidev driver contains lists of SPI devices that are supported for
> +the different hardware topology representations.
> +
> +The following are the SPI device tables supported by the spidev driver:
> +
> +    - struct spi_device_id spidev_spi_ids[]: list of devices that can be
> +      bound when these are defined using a struct spi_board_info with a
> +      .modalias field matching one of the entries in the table.
> +
> +    - struct of_device_id spidev_dt_ids[]: list of devices that can be
> +      bound when these are defined using a Device Tree node that has a
> +      compatible string matching one of the entries in the table.
> +
> +    - struct acpi_device_id spidev_acpi_ids[]: list of devices that can
> +      be bound when these are defined using a ACPI device object with a
> +      _HID matching one of the entries in the table.
> +
> +NOTE: it used to be supported to define an SPI device using the "spidev"
> +      name.  For example as .modalias = "spidev" or compatible = "spidev".
> +      But this is no longer supported by the Linux kernel and instead a
> +      real SPI device name as listed in one of the tables should be used.

This reads as the tables are fixed.
Perhaps add

    You are encouraged to add an entry for your SPI device name to
     one of the tables.

> +
>  Set up the other device characteristics (bits per word, SPI clocking,
>  chipselect polarity, etc) as usual, so you won't always need to override
>  them later.
>
> -(Sysfs also supports userspace driven binding/unbinding of drivers to
> -devices.  That mechanism might be supported here in the future.)
> +Sysfs also supports userspace driven binding/unbinding of drivers to
> +devices.  The mechanism works by writing to the device "driver_overrride"
> +entry.  For example:
> +
> +    echo spidev > /sys/bus/spi/devices/spiX.Y/driver_override
> +    echo spiB.C > /sys/bus/spi/drivers/spidev/bind
>
>  When you do that, the sysfs node for the SPI device will include a child
>  device node with a "dev" attribute that will be understood by udev or mdev.

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