Re: [PATCH v2 1/4] devicetree: bindings: Properly document micrel ks8851 SPI chips

[Rob Herring <robherring2@gmail.com>]

On Wed, May 28, 2014 at 10:16 AM, Uwe Kleine-König
<u.kleine-koenig@pengutronix.de> wrote:
> Hello Stephen,
>
> On Tue, May 27, 2014 at 02:40:15PM -0700, Stephen Boyd wrote:
>> On 05/24/14 05:48, Mark Brown wrote:
>> > On Fri, May 23, 2014 at 12:57:17PM -0700, Stephen Boyd wrote:
>> >
>> >>  Optional properties:
>> >> -- vdd-supply:     supply for Ethernet mac
>> >> +- vdd-supply: analog 3.3V supply for Ethernet mac
>> >> +- vdd-io-supply: digital 1.8V IO supply for Ethernet mac
>> > So, according to the datasheet I managed to find this device has a
>> > supply VDD_IO (so normally written vdd-io-supply here), some other
>> > supplies which are tied to VDD_IO (so can probably be omitted) and a
>> > supply VDD_A3.3 none of which are optional.  There is an internal
>> > regulator which can be used to drop a higher voltage VDD_IO down for
>> > some of the supplies tied to it but that's essentially a noop from
>> > software as far as I can tell.  None of these supplies are obviously
>> > optional, though I've not read the datasheet in detail so I may have
>> > missed something here.
> There is a difference between the supply being optional for the hardware
> to work and the need to specify it in the device tree, isn't it? My
> expectation is that when it's not specified there is just nothing the
> the software needs to care for.

Yes, agreed.

Of course you could have cases where a supply at the h/w level is
optional like if a supply can be powered externally or via an internal
regulator. Those cases will have to be made clear in the binding, but
a heading "Optional properties" in a binding doc means properties
which are optional to specify in DT.

Rob