On 11/1/14 4:11, Grant Likely wrote:
> On Tue, 28 Oct 2014 22:59:57 +0100
> , "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>
> wrote:
>> On Tuesday, October 28, 2014 01:15:27 PM Mika Westerberg wrote:
>>> acpi_dev_add_driver_gpios() makes it possible to set up mapping between
>>> properties and ACPI GpioIo resources in a driver, so we can take index
>>> parameter in acpi_find_gpio() into use with _DSD device properties now.
>>>
>>> This index can be used to select a GPIO from a property with multiple
>>> GPIOs:
>>>
>>> Package () {
>>> "data-gpios",
>>> Package () {
>>> \_SB.GPIO, 0, 0, 0,
>>> \_SB.GPIO, 1, 0, 0,
>>> \_SB.GPIO, 2, 0, 1,
>>> }
>>> }
>>>
>>> In order to retrieve the last GPIO from a driver we can simply do:
>>>
>>> desc = devm_gpiod_get_index(dev, "data", 2);
>>>
>>> and so on.
>>>
>>> Signed-off-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
>>
>> Cool. :-)
>>
>> Any objections anyone?
>
> Actually, I do. Not in the idea, but in the implementation. The way this gets encoded:
>
> Package () {
> \_SB.GPIO, 0, 0, 0,
> \_SB.GPIO, 1, 0, 0,
> \_SB.GPIO, 2, 0, 1,
> }
>
> Means that decoding each GPIO tuple requires the length of a tuple to be
> fixed, or to implement a DT-like #gpio-cells. If it is fixed, then there
> is no way to expand the binding later. Can this be done in the following
> way instead?
>
> Package () {
> Package () { \_SB.GPIO, 0, 0, 0 },
> Package () { \_SB.GPIO, 1, 0, 0 },
> Package () { \_SB.GPIO, 2, 0, 1 },
> }
>
> This is one of the biggest pains in device tree. We don't have any way
> to group tuples so it requires looking up stuff across the tree to
> figure out how to parse each multi-item property.
>
> I know that last year we talked about how bios vendors would get
> complicated properties wrong, but I think there is little risk in this
> case. If the property is encoded wrong, the driver simply won't work and
> it is unlikely to get shipped before being fixed.
This particular nesting of Packages is expressly prohibited by the
Device Properties UUID for the reasons you mention.
http://www.uefi.org/sites/default/files/resources/_DSD-device-properties-UUID.pdf
2.1 Data Format Definition
...
" a Package consisting entirely of Integer, String, or Reference objects
(and
specifically not containing a nested Package)."
That said, I am not fond of the many properties mixed in as a single
Package. We discussed this at some length while this was being proposed,
and this was deemed the lesser evil.
--
Darren Hart
Intel Open Source Technology Center
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