linux-kernel - Re: [PATCH 4/5] HID: autoload hid-multitouch as needed

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Date:	Thu, 8 Mar 2012 12:30:09 +0100
From:	"Henrik Rydberg" <rydberg@...omail.se>
To:	Stéphane Chatty <chatty@...c.fr>
Cc:	Jiri Kosina <jkosina@...e.cz>,
	"benjamin.tissoires" <benjamin.tissoires@...il.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] HID: autoload hid-multitouch as needed

Hi Stéphane,

> > What if we were to change the definition of a HID device on the
> > modalias level?
> > 
> > In practise, a HID device can be either an usb device, a hid device,
> 
> Just to be sure: do you mean "bluetooth device"? or is there such a
> thing as a hid device per se? I'm asking because I've always been
> surprised at seeing usbhid/ in hid/, which kind of breaks the
> potential symmetry between USB and Bluetooth wrt hid.

Oops, yes, I meant bluetooth devices.

> > a single interface on a usb bus, a special class determined by examining
> > the reports, etc. Yet, the hid modalias contains only bus type, vendor
> > and product id. This is true for the generic usb and bluetooth drivers
> > (and some very special drivers), but not really for the other devices.
> > If we were to extend the modalias description, we could cater for a
> > whole tree of hid devices. For instance, the usb id 1234 could be
> > handled by the generic usb bus driver. The multitouch sub-device
> > 1234:MT could be handled by hid-multitouch. The mouse device
> > 1234:Mouse could be handled by some other driver, etc. All the driver
> > handling could be automated in userland using the same udev mechanism
> > we have today, if only the hid uevents were modified to incorporate
> > the needed extra information.
> 
> No comments on this, I need to read up on modaliases before being
> able to comment at all. I have a vague feeling that we are going to
> end up debating where it is decided to assign a device to a driver
> (today it's done in the kernel, you seem to suggest userland), but I
> know too little about modaliases to be sure.

The device-driver matching is done in the kernel, but driver (aka
module) loading is done from userland. The crux is to be able to tell
userland what driver to load for a certain device. In this case, it
means giving more information to userland via the device/modalias
construct. Or at least, that is the question. :-)

Thanks,
Henrik
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