Re: Support for Logitech MX Anywhere 2

[Peter Hutterer <peter.hutterer@who-t.net>]

On Sat, Mar 25, 2017 at 01:02:10PM -0300, Mauro Carvalho Chehab wrote:
> Em Sat, 25 Mar 2017 09:36:18 -0300
> Mauro Carvalho Chehab <mchehab@s-opensource.com> escreveu:
> 
> > Em Fri, 24 Mar 2017 06:57:00 -0300
> > Mauro Carvalho Chehab <mchehab@s-opensource.com> escreveu:
> > 
> > > Em Fri, 24 Mar 2017 15:22:20 +1000
> > > Peter Hutterer <peter.hutterer@who-t.net> escreveu:
> > >   
> > > > On Thu, Mar 23, 2017 at 02:29:00PM -0300, Mauro Carvalho Chehab wrote:    
> > > > > Em Thu, 23 Mar 2017 11:59:56 +0100
> > > > > Benjamin Tissoires <benjamin.tissoires@redhat.com> escreveu:      
> > > > > > > With regards to ratchet, it probably makes sense to query its state
> > > > > > > when the driver starts, as IMHO, it should work on a way similar to
> > > > > > > <CAPS LOCK>. Btw, are there any event already defined for ratchet mode?        
> > > > > > 
> > > > > > There is not. And that's where the problem goes a little bit beyond just
> > > > > > enabling the feature, we need to forward this info to userspace.
> > > > > > 
> > > > > > There should be some EV_SWITCH SW_RATCHET created IMO. The ratchet has
> > > > > > a state, and we should be able to forward this state with such a new
> > > > > > event.
> > > > > > 
> > > > > > The thing I am more worried is how can we report the high-res wheel
> > > > > > events. I know Peter has a DB of wheel resolution, but in that case, we
> > > > > > can switch to high-res or not, so a static hwdb entry won't help.      
> > > > > 
> > > > > Yes. In the case of high-res, there are actually two ways for those
> > > > > events to arrive:
> > > > > 
> > > > > In HID mode, it produces standard EV_REL / REL_WHEEL events, but there
> > > > > isn't any events reporting if the mode changed, nor the event with
> > > > > gets wheel axes movement tell if the mouse is in low or high res.
> > > > > 
> > > > > So, in HID mode, identifying if the wheel is in low or high res
> > > > > is not direct.
> > > > > 
> > > > > When the device is in HID+ mode, the resolution mode comes together with
> > > > > axes changes. So, it should be possible to define a different event
> > > > > for high-res wheel.
> > > > > 
> > > > > E. g. if the event reports high-res, it would be generating:
> > > > > EV_REL / REL_HI_RES_WHEEL. If the packet arrives with low-res,
> > > > > it will keep generating EV_REL / REL_WHEEL.
> > > > > 
> > > > > This way, Peter's code (libinput, I guess?) could handle it without
> > > > > requiring any DB for the devices that allow switching the mode.      
> > > > 
> > > > sort-of. The main problem with relative axes is that we don't have a
> > > > resolution info. The reason we have a hwdb for wheels is that libinput
> > > > converts kernel data to physical dimensions so that callers can use the data
> > > > in a reliable manner. Switching to a hi-res-wheel just moves the problem
> > > > around a bit.
> > > > 
> > > > Using ABS events simply gives us the resolution in the inital description.
> > > > That's (I suspect) the only reason Benjamin suggested it. This isn't the
> > > > first time it has come up, it would be interesting to add something like
> > > > EVIOCGREL as equivalent to EVIOCGABS and start augmenting rel data with
> > > > resolution. But I also suspect that all but this use-case would have the
> > > > kernel return a digital shrug anyway, so I'm not sure it's worth the effort.    
> > > 
> > > I see. Well, at least in the case of the feature supported by this
> > > mouse, there are just two possible resolutions: low-res and high-res.
> > > The high-res resolution is fixed[1].
> > > 
> > > As the multiplier has a fixed value per device, a hwdb could still
> > > work, provided that high-res wheel events would produce a different
> > > event code than low-res.
> > > 
> > > [1] there's a USB message that can be used to query the multiplier,
> > > with is always equal to 8 for MX Anywhere 2. No idea if other
> > > devices with this feature use the same multiplier.  

let's not assume that and plan ahead, because sooner or later this will be
configurable in some device. Probably before we get the first kernel out
with this patchset in. :)

a hwdb could still work in that case, but it gets quite tricky when it
becomes user-configurable. Now you need the user (or supporting software) to
drop hwdb in entries which is less than ideal. If there is a value that can
be queried from the device, we should figure out how to use it.

> > What I'm proposing is basically something like what's in the patch
> > below (for now, just compile-tested).
> > 
> > So, for MX Anywhere 2 and MX master, the hid-logitech-hidpp driver should
> > switch to the HID++ report mode at device connect and handle the Wheel
> > events. If the wheel event is low res, will generate
> > REL_WHEEL events. if the wheel is in high resolution, REL_HWHEEL.
> 
> Hmm... "H" in HWHEEL is not for "hardware" or "high-res", but,
> instead, for "horizontal".

yep :)

> So, if we'd go for the proposal of using a different event for high-res
> vertical wheel, instead of adding a new ioctl (EVIOCGREL as equivalent to
> EVIOCGABS), we'll need an extra event.
> 
> Anyway, the patch below works fine with my mouse. It is against
> Kernel 4.10.4.

quick skim of the patch looks ok, but the big issue with this isn't the
technical bit but the policy bit. So the last hunk adding the event code
should accompanied by a Documentation/input/event-codes.txt hunk, explaining
the use of the code and how it interacts with the other event codes.
You'll probably find that as you write that documentation, more
questions will come up.

Cheers,
   Peter