Date: Tue, 21 Jan 2014 08:56:51 +1000
From: Peter Hutterer <peter.hutterer@who-t.net>
To: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Benjamin Tissoires <benjamin.tissoires@redhat.com>,
        David Herrmann <dh.herrmann@gmail.com>,
        linux-input <linux-input@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3] input/uinput: add UI_GET_SYSNAME ioctl to retrieve
 the sysfs path
Message-ID: <20140120225651.GA30049@yabbi.local>
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 <CAN+gG=EPXLR5H_h0UgaMOmAbwAivxCohtg3Gocws5fJucz4+eg@mail.gmail.com>
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On Mon, Jan 20, 2014 at 05:17:08PM -0500, Benjamin Tissoires wrote:
> On Mon, Jan 20, 2014 at 4:53 PM, Dmitry Torokhov
> <dmitry.torokhov@gmail.com> wrote:
> > Hi Benjamin,
> >
> > On Fri, Jan 17, 2014 at 02:12:51PM -0500, Benjamin Tissoires wrote:
> >> Evemu [1] uses uinput to replay devices traces it has recorded. However,
> >> the way evemu uses uinput is slightly different from how uinput is
> >> supposed to be used.
> >> Evemu relies on libevdev, which creates the device node through uinput.
> >> It then injects events through the input device node directly (and it
> >> completely skips the uinput node).
> >>
> >> Currently, libevdev relies on an heuristic to guess which input node was
> >> created. The problem is that is heuristic is subjected to races between
> >> different uinput devices or even with physical devices. Having a way
> >> to retrieve the sysfs path allows us to find the event node without
> >> having to rely on this heuristic.
> >
> > I have been thinking about it and I think that providing tight coupling
> > between uinput and resulting event device is wrong thing to do. We do
> > allow sending input events through uinput interface and I think evemu
> > should be using it, instead of going halfway through uinput and halfway
> > though evdev. Replaying though uinput would actually be more correct as
> > it would involve the same code paths throgugh input core as with using
> > real devices (see input_event() vs. input_inject_event() that is used by
> > input handlers).
> >
> 
> Yes, I am perfectly aware of the fact that evemu is not using uinput
> in the way it is intended to be.
> I agree that it should be using the uinput node to inject events but
> this means that only the process which has created the virtual device
> can access it. It seems weird, I know, but the typical use of evemu is
> the following:
> - in a first terminal: $> sudo evemu-device mydevice.desc
> - In a second: $> sudo evemu-play /dev/input/event12 < mydevice.events
> 
> It looks weird here, but it allows to inject different events
> recording for the same virtual device node. 

it also allows replaying an event through the device it was recorded on.
it's not always necessary or desirable to create a uinput device, sometimes
replaying it through the actual device is better to reproduce a certain bug.

Cheers,
   Peter


> Using the uinput node to
> inject events will force us to change the user "interface" and rely on
> pipes to get the same separation of describe/inject.
> Note that I am modifying evemu-play to be able to also create the
> virtual device, so I am not entirely convinced about this argument
> (but we have users).
> 
> The other use case I should have mentioned in the commit message is
> that we extensively rely on evemu for the xorg-integration-tests (and
> the upcoming wayland test suite if I am not wrong).
> The tests are fully automatized, and we need to know which input node
> has just been created to record the correct one and test against it.
> 
> 
> Ok, I am stopping here because Peter already answered about this in his mail :)
> 
> Cheers,
> Benjamin
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