linux-kernel - Re: [PATCH 0/3] Fixes for multitouch / generic gamepads interaction

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Date:	Thu, 21 Nov 2013 13:38:30 +0100
From:	Henrik Rydberg <rydberg@...omail.se>
To:	Benjamin Tissoires <benjamin.tissoires@...hat.com>,
	Benjamin Tissoires <benjamin.tissoires@...il.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	David Herrmann <dh.herrmann@...il.com>,
	Antonio Ospite <ospite@...denti.unina.it>, simon@...gewell.org,
	case@...psmoderne.net, linux-input@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] Fixes for multitouch / generic gamepads interaction

Hi Benjamin,

> I have been reported recently a problem with the Sixaxis controller
> (http://www.spinics.net/lists/linux-input/msg28098.html).
> The root of the problem comes from hid-input, which maps unknown axis to
> ABS_MISC. However, when an event code is already in use, hid-input uses the one
> after, leading to uses of ABS_MISC + N, where N is the number of unknown axis.

Yes, and clearly we should not do that. :-)

> We are encountering a problem with the multitouch protocol here because if a
> device has more than 7 unknown axis (which is the case for the PS3 Sixaxis
> controller), then the unknown axis get maps to ABS_MT_SLOT and beyond.
> 
> This infers two problems:
> - the axis currently mapped on ABS_MT_SLOT is a special case in the kernel,
> and it is not updated
> - the axis after ABS_MT_SLOT are not filtered anymore, as the kernel things
> the device is using a multitouch protocol A.

The problem is generic, and ABS_MT_SLOT just happens to be the first value that
the hid logic hits. When sevenaxis comes out, we have to do this all over again.

> The patch 0001 solves the first problem, whereas the patches 0002 and 0003
> fix the second. Bonus point: the userspace is now correctly notified that one of
> the multitouch protocols is in used, so it does not have to rely on bad designed
> heuristics.

The bad heuristics are lines 918 and onwards in hid-input.c. I am sure we can
come up with a way to allocate those axes in a much less fragile way.

Thanks,
Henrik

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