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

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:

quoted

On Nov 30 2016 or thereabouts, Brian Norris wrote:

quoted

From: Caesar Wang <redacted>

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 <redacted>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: linux-input@vger.kernel.org
Signed-off-by: Brian Norris <briannorris@chromium.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.

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].

Now if you want to make 'hid-over-i2c' a fallback to 'wacom,w9013', I'm 
fine with that.

Rob

[1] https://sietch-tagr.blogspot.de/2016/04/display-panels-are-not-special.html