Re: [PATCH v1 6/8] gpio: acpi: Explain how to get GPIO descriptors in ACPI case

[Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>]

On Thu, Mar 23, 2017 at 09:46:16PM +0200, Andy Shevchenko wrote:
> Documentation lacks of explanation how we actually use device properties
> for GPIO resources.
> 
> Add a section to the documentation about that.
> 
> Suggested-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> ---
>  Documentation/acpi/gpio-properties.txt | 60 ++++++++++++++++++++++++++++++++++
>  1 file changed, 60 insertions(+)
> 
> diff --git a/Documentation/acpi/gpio-properties.txt b/Documentation/acpi/gpio-properties.txt
> index 2aff0349facd..07954b7c3a12 100644
> --- a/Documentation/acpi/gpio-properties.txt
> +++ b/Documentation/acpi/gpio-properties.txt
> @@ -156,3 +156,63 @@ pointed to by its first argument.  That should be done in the driver's .probe()
>  routine.  On removal, the driver should unregister its GPIO mapping table by
>  calling acpi_dev_remove_driver_gpios() on the ACPI device object where that
>  table was previously registered.
> +
> +Using the _CRS fallback
> +-----------------------
> +
> +If a device does not have _DSD or the driver does not create ACPI GPIO
> +mapping, the Linux GPIO framework refuses to return any GPIOs. This is
> +because the driver does not know what it actually gets. For example if we
> +have a device like below:
> +
> +  Device (BTH)
> +  {
> +      Name (_HID, ...)
> +
> +      Name (_CRS, ResourceTemplate () {
> +          GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionNone,
> +                  "\\_SB.GPO0", 0, ResourceConsumer) {15}
> +          GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionNone,
> +                  "\\_SB.GPO0", 0, ResourceConsumer) {27}
> +      })
> +  }
> +
> +The driver might expect to get the right GPIO when it does:
> +
> +  desc = gpiod_get(dev, "reset", GPIOD_OUT_LOW);
> +
> +but since there is no way to know the mapping between "reset" and
> +the GpioIo() in _CRS desc will hold ERR_PTR(-ENOENT).
> +
> +The driver author can solve this by passing the mapping explictly
> +(the recommended way and documented in the above chapter).

If the driver is not platform specific, then it would have no idea about
mapping between _CRS GPIOs and names. All such stuff should be hidden in
platform glue (i.e drivers/platform/x86/platform_crap.c).

> +
> +Getting GPIO descriptor
> +-----------------------
> +
> +There are two main approaches to get GPIO resource from ACPI:
> +	desc = gpiod_get(dev, connection_id, flags);
> +	desc = gpiod_get_index(dev, connection_id, index, flags);
> +
> +We may consider two different cases here, i.e. when connection ID is
> +provided and otherwise.
> +
> +Case 1:
> +	desc = gpiod_get(dev, "non-null-connection-id", flags);
> +	desc = gpiod_get_index(dev, "non-null-connection-id", index, flags);
> +
> +Case 2:
> +	desc = gpiod_get(dev, NULL, flags);
> +	desc = gpiod_get_index(dev, NULL, index, flags);
> +
> +Case 1 assumes that corresponding ACPI device description must have
> +defined device properties and will prevent to getting any GPIO resources
> +otherwise.
> +
> +Case 2 explicitly tells GPIO core to look for resources in _CRS.
> +
> +Be aware that gpiod_get_index() in cases 1 and 2, assuming that there
> +are two versions of ACPI device description provided and no mapping is
> +present in the driver, will return different resources. That's why a
> +certain driver has to handle them carefully as explained in previous
> +chapter.

I think that this wording is too x86-centric. We are talking about
consumers of GPIOs here (i.e. drivers), which need unified behavior
between ACPI, DT, and static board properties, they do not really care
about _CRS or _DSD.

Thanks.

-- 
Dmitry
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