Date:   Thu, 08 Dec 2016 08:13:41 -0800
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Rob Herring <robh@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>
CC:     Doug Anderson <dianders@...omium.org>,
        Brian Norris <briannorris@...omium.org>,
        Jiri Kosina <jikos@...nel.org>,
        Caesar Wang <wxt@...k-chips.com>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        "linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v2 1/2] devicetree: i2c-hid: Add Wacom digitizer + regulator support

On December 8, 2016 8:03:06 AM PST, Rob Herring <robh@...nel.org> wrote:
>On Thu, Dec 8, 2016 at 9:41 AM, Benjamin Tissoires
><benjamin.tissoires@...hat.com> wrote:
>> On Dec 06 2016 or thereabouts, Doug Anderson wrote:
>>> Hi,
>>>
>>> On Tue, Dec 6, 2016 at 6:56 AM, Rob Herring <robh@...nel.org> wrote:
>>> > On Tue, Dec 6, 2016 at 2:48 AM, Benjamin Tissoires
>>> > <benjamin.tissoires@...hat.com> wrote:
>>> >> On Dec 05 2016 or thereabouts, Rob Herring wrote:
>>> >>> On Thu, Dec 01, 2016 at 09:24:50AM -0800, Brian Norris wrote:
>>> >>> > Hi Benjamin and Rob,
>>> >>> >
>>> >>> > On Thu, Dec 01, 2016 at 03:34:34PM +0100, Benjamin Tissoires
>wrote:
>>> >>> > > On Nov 30 2016 or thereabouts, Brian Norris wrote:
>>> >>> > > > From: Caesar Wang <wxt@...k-chips.com>
>>> >>> > > >
>>> >>> > > > Add a compatible string and regulator property for Wacom
>W9103
>>> >>> > > > digitizer. Its VDD supply may need to be enabled before
>using it.
>>> >>> > > >
>>> >>> > > > Signed-off-by: Caesar Wang <wxt@...k-chips.com>
>>> >>> > > > Cc: Rob Herring <robh+dt@...nel.org>
>>> >>> > > > Cc: Jiri Kosina <jikos@...nel.org>
>>> >>> > > > Cc: linux-input@...r.kernel.org
>>> >>> > > > Signed-off-by: Brian Norris <briannorris@...omium.org>
>>> >>> > > > ---
>>> >>> > > > v1 was a few months back. I finally got around to
>rewriting it based on
>>> >>> > > > DT binding feedback.
>>> >>> > > >
>>> >>> > > > v2:
>>> >>> > > >  * add compatible property for wacom
>>> >>> > > >  * name the regulator property specifically (VDD)
>>> >>> > > >
>>> >>> > > >  Documentation/devicetree/bindings/input/hid-over-i2c.txt
>| 6 +++++-
>>> >>> > > >  1 file changed, 5 insertions(+), 1 deletion(-)
>>> >>> > > >
>>> >>> > > > diff --git
>a/Documentation/devicetree/bindings/input/hid-over-i2c.txt
>b/Documentation/devicetree/bindings/input/hid-over-i2c.txt
>>> >>> > > > index 488edcb264c4..eb98054e60c9 100644
>>> >>> > > > ---
>a/Documentation/devicetree/bindings/input/hid-over-i2c.txt
>>> >>> > > > +++
>b/Documentation/devicetree/bindings/input/hid-over-i2c.txt
>>> >>> > > > @@ -11,12 +11,16 @@ If this binding is used, the kernel
>module i2c-hid will handle the communication
>>> >>> > > >  with the device and the generic hid core layer will
>handle the protocol.
>>> >>> > > >
>>> >>> > > >  Required properties:
>>> >>> > > > -- compatible: must be "hid-over-i2c"
>>> >>> > > > +- compatible: must be "hid-over-i2c", or a
>device-specific string like:
>>> >>> > > > +    * "wacom,w9013"
>>> >>> > >
>>> >>> > > NACK on this one.
>>> >>> > >
>>> >>> > > After re-reading the v1 submission I realized Rob asked for
>this change,
>>> >>> > > but I strongly disagree.
>>> >>> > >
>>> >>> > > HID over I2C is a generic protocol, in the same way HID over
>USB is. We
>>> >>> > > can not start adding device specifics here, this is opening
>the can of
>>> >>> > > worms. If the device is a HID one, nothing else should
>matter. The rest
>>> >>> > > (description of the device, name, etc...) is all provided by
>the
>>> >>> > > protocol.
>>> >>> >
>>> >>> > I should have spoken up when Rob made the suggestion, because
>I more or
>>> >>> > less agree with Benjamin here. I don't really see why this
>needs to have
>>> >>> > a specialized compatible string, as the property is still
>fairly
>>> >>> > generic, and the entire device handling is via a generic
>protocol. The
>>> >>> > fact that we manage its power via a regulator is not very
>>> >>> > device-specific.
>>> >>>
>>> >>> It doesn't matter that the protocol is generic. The device
>attached and
>>> >>> the implementation is not. Implementations have been known to
>have
>>> >>> bugs/quirks (generally speaking, not HID over I2C in
>particular). There
>>> >>> are also things outside the scope of what is 'hid-over-i2c' like
>what's
>>> >>> needed to power-on the device which this patch clearly show.
>>> >>
>>> >> Yes, there are bugs, quirks, even with HID. But the HID declares
>within
>>> >> the protocol the Vendor ID and the Product ID, which means once
>we pass
>>> >> the initial "device is ready" step and can do a single i2c
>write/read,
>>> >> we don't give a crap about device tree anymore.
>>> >>
>>> >> This is just about setting the device in shape so that it can
>answer a
>>> >> single write/read.
>>> >>
>>> >>>
>>> >>> This is no different than a panel attached via LVDS, eDP, etc.,
>or
>>> >>> USB/PCIe device hard-wired on a board. They all use standard
>protocols
>>> >>> and all need additional data to describe them. Of course, adding
>a
>>> >>> single property for a delay would not be a big deal, but it's
>never
>>> >>> ending. Next you need multiple supplies, GPIO controls, mutiple
>>> >>> delays... This has been discussed to death already. As Thierry
>Reding
>>> >>> said, you're not special[1].
>>> >>
>>> >> I can somewhat understand what you mean. The official
>specification is
>>> >> for ACPI. And ACPI allows to calls various settings while
>querying the
>>> >> _STA method for instance. So in the ACPI world, we don't need to
>care
>>> >> about regulators or GPIOs because the OEM deals with this in its
>own
>>> >> blob.
>>> >>
>>> >> Now, coming back to our issue. We are not special, maybe, if he
>says so.
>>> >> But this really feels like a design choice between putting the
>burden on
>>> >> device tree and OEMs or in the module maintainers. And I'd rather
>have
>>> >> the OEM deal with their device than me having to update the
>module for
>>> >> each generations of hardware. Indeed, this looks like an
>"endless"
>>> >> amount of quirks, but I'd rather have this endless amount of
>quirks than
>>> >> having to maintain an endless amount of list of new devices that
>behaves
>>> >> the same way. We are talking here about "wacom,w9013", but then
>comes
>>> >> "wacom,w9014" and we need to upgrade the kernel.
>>> >
>>> > No. If the w9014 can claim compatibility with then w9013, then you
>>> > don't need a kernel change. The DT should list the w9014 AND
>w9013,
>>> > but the kernel only needs to know about the w9013. That is until
>there
>>> > is some difference which is why the DT should list w9014 to start
>>> > with.
>>> >
>>> > If you don't have any power control to deal with, then the kernel
>can
>>> > always just match on "hid-over-i2c" compatible.
>>>
>>> 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.
>>>
>>> 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.
>>>
>>> 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.
>>>
>>
>> Thanks Doug for this. I had the feeling this wasn't right, but you
>> actually managed to put the words on it. If it's a board problem (if
>> you switch the wacom device with an other HID over I2C device and you
>> still need the same regulator/timing parameters), then this should
>> simply be mentioned on the patch.
>>
>> So Brian, could you please respin the series and remve the Wacom
>> mentions and explain that it is required for the board itself?
>
>In advance, NAK.
>
>This is not how DT works. Either this binding needs a Wacom compatible
>or don't use DT.
>

And if tomorrow there is Elan device that is drop-in compatible (same connector, etc) with Wacom i2c-hid, will you ask for Elan-specific binding? Atmel? Weida? They all need to be powered up ultimately.


Thanks.

-- 
Dmitry

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