Date:	Tue, 30 Jun 2015 17:26:23 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	Peter Hutterer <peter.hutterer@...-t.net>,
	Hans de Goede <hdegoede@...hat.com>,
	Henrik Rydberg <rydberg@...math.org>
Subject: Re: [PATCH 2/2] Input - synaptics: pin 3 touches when the firmware
 reports 3 fingers

Hi Benjamin,

On Thu, Jun 11, 2015 at 01:29:09PM -0400, Benjamin Tissoires wrote:
> On Apr 24 2015 or thereabouts, Benjamin Tissoires wrote:
> > Hi Dmitry,
> > 
> > [ adding more relevant people to the discussion ]
> > 
> > On Apr 23 2015 or thereabouts, Benjamin Tissoires wrote:
> > > On Apr 23 2015 or thereabouts, Dmitry Torokhov wrote:
> > > > On Wed, Apr 22, 2015 at 11:45:09AM -0400, Benjamin Tissoires wrote:
> > > > > Synaptics PS/2 touchpad can send only 2 touches in a report. They can
> > > > > detect 4 or 5 and this information is valuable.
> > > > > 
> > > > > In commit 63c4fda3c0bb ("Input: synaptics - allocate 3 slots to keep
> > > > > stability in image sensors"), we allocate 3 slots, but we still continue
> > > > > to report the 2 available fingers. That means that the client sees 2 used
> > > > > slots while there is a total of 3 fingers advertised by BTN_TOOL_TRIPLETAP.
> > > > > 
> > > > > For old kernels this is not a problem because max_slots was 2 and libinput/
> > > > > xorg-synaptics knew how to deal with that. Now that max_slot is 3, the
> > > > > clients ignore BTN_TOOL_TRIPLETAP and count the actual used slots (so 2).
> > > > > It then gets confused when receiving the BTN_TOOL_TRIPLETAP and DOUBLETAP
> > > > > information, and goes wild.
> > > > > 
> > > > > We can pin the 3 slots until we get a total number of fingers below 2.
> > > > > 
> > > > > Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1212230
> > > > 
> > > > Benjamin, I do not quite like it. It seems that original patch was not
> > > > quite right and we are adding more workarounds.
> > > 
> > > Agree. And I am starting to hate more and more the synaptics PS/2 and all
> > > the PS/2 drivers to be honest :) - trying to fit a heavy load data like
> > > multitouch in a simple and lightweight protocol like PS/2 is insane...
> > > 
> > > We are internally trying to figure out if we can finally take advantage
> > > of the SMBus/RMI4 protocol, but we tried for one year without much
> > > success.
> > > 
> > > > 
> > > > Synaptics can only track 2 contacts, correct? Why 2 slots to track them
> > > > is not enough?
> > > 
> > > IIRC, the problem was that upon a third finger down, with only 2 slots,
> > > the fingers were silently inverted in most cases. The thing is that the
> > > firmware forwards 2 fingers, but not necessarily the two first. So you
> > > generally get fingers 1+3 so the slot 2 needs to be removed. And that
> > > means the kernel tracking has to track 3 fingers upon transitions.
> > > 
> > > This may be completely bullshit and we might not need to use 3 slots at
> > > all. I'll need to do further experiments to validate which one is best
> > > then.
> > > 
> > > I am perfectly fine holding this one up for a little bit more testings
> > > and then we can decide which one needs to be done (revert or an other
> > > band-aid).
> > > 
> > 
> > So I carefully recorded each situation (initial with 2 slots, 2 slots
> > and then with the pinning in this patch*), and I am now convinced that
> > the pinning is the best sequence that we forward to the user space (best
> > among the 3).
> > 
> > With 2 slots declared, there are 2 problems:
> > - the first finger jumps to the position of the 3rd when it lands
> > - the transition between 2 to 3 fingers goes to a state where the kernel
> >   removes the second finger (while jumping the first to the position of
> >   the 3rd finger), send a sync and then reallocate the first finger
> >   position as the second slot in use
> > 
> > -> that means that user space sees a small transition where the slots
> > count is 1 while the BTN_TOOL advertise triple tap :/
> > 
> > With 3 slots, we have the problem reported in the rhbz bug  #1212230:
> > - during the transition, the fingers are stable, but we have at most 2
> >   active slots in one frame, which confuses libinput/xorg-synaptics.
> > 
> > With the pinning, the user space is no more confused because BTN_TOOL is
> > always greater or equal than the active slots.
> > 
> > So I think for now we have 3 possibilities:
> > 1. Just carry this patch, and hope that we will be able to switch the
> >    synaptics device in the non-PS/2 mode
> > 2. Revert to 2 patches and fix the kernel tracking to accept 3 fingers
> >    and return the 2 best matches
> > 3. Revert the use of the kernel tracking at all and re-introduce the
> >    spaghetti code that was here before and hope that all cases where
> >    properly handled.
> > 
> > IMO that the solution 2. is the best, but I can not do it because I
> > don't understand what the code does. I can guess things but I can not
> > accurately change it because it is not readable IMO.
> > 
> > (yes, there is also the solution 4: "screw up and let the user space deal
> > with it", but I'd rather not do that given the history of the multitouch
> > protocol)
> 
> Dmitry, I feel like this discussion fell a little bit between the cracks
> and that we all forgot about it.
> 
> I still believe that the patch is needed (even if it is not the best
> solution), so I am sending a gently ping on this one :)

Sorry I lost track of this, but I still believe that introducing the 3rd
slot is not the right solution as is evidenced by the need of more
workarounds. If the hardware is only capable of tracking the 2 contacts
then we should be using 2 slots. It seems that userspace (and maybe the
kernel as well?) is not quite prepared to handle change of contact's
identity in a slot (i.e. assigning new tracking id to a slot without
transitioning through -1), but that is what we need to fix then.

I think we should revert 63c4fda3c0bb. 

Thanks.

-- 
Dmitry
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