Re: [PATCH v2 1/2] devicetree: i2c-hid: Add Wacom digitizer + regulator support

[Doug Anderson]

Hi,

On Thu, Dec 8, 2016 at 8:01 AM, Rob Herring <robh@xxxxxxxxxx> wrote:
>> Just my $0.02.  Feel free to ignore.
>>
>> One thought is that I would say that the need to power on the device
>> explicitly seems more like a board level difference and less like a
>> difference associated with a particular digitizer.  Said another way,
>> it seems likely there will be boards with a w9013 without explicit
>> control of the regulator in software and it seems like there will be
>> boards with other digitizers where suddenly a new board will come out
>> that needs explicit control of the regulator.
>
> Then either the regulator is optional or you don't say it is a w9013
> for that board. But if you do need to initialize the device and
> therefore know what type of device it is, then you need a compatible
> for the device. It's when things really vary by board that we add DT
> properties.
>
>> In this particular case I feel like we can draw a lot of parallels to
>> an SDIO bus.
>>
>> When you specify an SDIO bus you don't specify what kind of card will
>> be present, you just say "I've got an SDIO bus" and then the specific
>> device underneath is probed.  Here we've say "I've got an i2c
>> connection intended for HID" and then you probe for the HID device
>> that's on the connection.
>
> No, the soldered down devices require all sorts of extra non-SDIO
> connections and we do specify the device in those cases.

We have never specified the device on boards I've worked with.

On rk3288-veyron, for instance, we might have stuffed a Broadcom 4354
WiFi chip or a Marvell 8897 WiFi chip.  Some veyron boards have one
chip and some have the other.  ...and during bringup we even had some
of the exact same boards where half were stuffed with one chip and
half with the other.

Nothing in the device tree says which chip is stuffed.  In both cases
the board uses the same power on sequence for the WiFi chip.  Once
that is done, we dynamically probe which actual WiFi part is stuffed.

Certainly not all users that have these WiFi chips use the same power
on sequence.  I have certainly seen development boards for these chips
where you just insert them into a regular SD card slot.  This is a
more expensive solution because you need more logic on the board, but
it shows that the power on sequence is not associated with these
chips.


>> Also for an SDIO bus, you've possibly got a regulators / GPIOs /
>> resets that need to be controlled, but the specific details of these
>> regulator / GPIOs / resets are specific to a given board and not
>> necessarily a given SDIO device.
>
> It's both. The device defines what is needed and the specs to control
> them (active states of GPIOs, de/assertion times of resets, supply
> voltages, etc.). The board only determines what the connections are
> and if you can control them.

It's not always that simple.  The device says that it needs power and
resets to happen.  How power is provided and how resets happen is
awfully board specific.  As per above it is possible that the board
wouldn't need to be involved above is you want to spend more money /
power.

-Doug