Re: [PATCH 2/2] Input - synaptics: pin 3 touches when the firmware reports 3 fingers

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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)


Cheers,
Benjamin


* I tried to put side by side the 3 test cases in the following logs:


(init slots 2, no pinning)    | (init slots 3, no pinning)    | (init slots 3, pinning)
----------------------------- | ----------------------------- | ---------------------------
------------------------------|-----  one finger down  -------|----------------------------
                              |                               |
ABS_MT_SLOT          0        | ABS_MT_SLOT          0        | ABS_MT_SLOT          0
ABS_MT_TRACKING_ID   53       | ABS_MT_TRACKING_ID   53       | ABS_MT_TRACKING_ID   53
ABS_MT_POSITION_X    2000     | ABS_MT_POSITION_X    2000     | ABS_MT_POSITION_X    2000
ABS_MT_POSITION_Y    2000     | ABS_MT_POSITION_Y    2000     | ABS_MT_POSITION_Y    2000
ABS_MT_PRESSURE      68       | ABS_MT_PRESSURE      68       | ABS_MT_PRESSURE      68
BTN_TOUCH            1        | BTN_TOUCH            1        | BTN_TOUCH            1
ABS_X                2000     | ABS_X                2000     | ABS_X                2000
ABS_Y                2000     | ABS_Y                2000     | ABS_Y                2000
ABS_PRESSURE         68       | ABS_PRESSURE         68       | ABS_PRESSURE         68
BTN_TOOL_FINGER      1        | BTN_TOOL_FINGER      1        | BTN_TOOL_FINGER      1
--- SYN_REPORT (0) ---------- | --- SYN_REPORT (0) ---------- | --- SYN_REPORT (0) ----------
                              |                               |
...                           | ...                           | ...
                              |                               |
------------------------------------  second finger down ------------------------------------
                              |                               |
--- SYN_REPORT (0) ---------- | --- SYN_REPORT (0) ---------- | --- SYN_REPORT (0) ----------
ABS_MT_POSITION_X    2000     | ABS_MT_POSITION_X    2000     | ABS_MT_POSITION_X    2000
ABS_MT_POSITION_Y    2000     | ABS_MT_POSITION_Y    2000     | ABS_MT_POSITION_Y    2000
ABS_MT_PRESSURE      78       | ABS_MT_PRESSURE      78       | ABS_MT_PRESSURE      78
ABS_MT_SLOT          1        | ABS_MT_SLOT          1        | ABS_MT_SLOT          1
ABS_MT_TRACKING_ID   54       | ABS_MT_TRACKING_ID   54       | ABS_MT_TRACKING_ID   54
ABS_MT_POSITION_X    3000     | ABS_MT_POSITION_X    3000     | ABS_MT_POSITION_X    3000
ABS_MT_POSITION_Y    3000     | ABS_MT_POSITION_Y    3000     | ABS_MT_POSITION_Y    3000
ABS_MT_PRESSURE      64       | ABS_MT_PRESSURE      64       | ABS_MT_PRESSURE      64
ABS_X                2000     | ABS_X                2000     | ABS_X                2000
ABS_Y                2000     | ABS_Y                2000     | ABS_Y                2000
ABS_PRESSURE         78       | ABS_PRESSURE         78       | ABS_PRESSURE         78
BTN_TOOL_FINGER      0        | BTN_TOOL_FINGER      0        | BTN_TOOL_FINGER      0
BTN_TOOL_DOUBLETAP   1        | BTN_TOOL_DOUBLETAP   1        | BTN_TOOL_DOUBLETAP   1
--- SYN_REPORT (0) ---------- | --- SYN_REPORT (0) ---------- | --- SYN_REPORT (0) ----------
                              |                               |
...                           | ...                           | ...
                              |                               |
------------------------------------  third finger down ------------------------------------
                              |                               |
--- SYN_REPORT (0) ---------- | --- SYN_REPORT (0) ---------- | --- SYN_REPORT (0) ----------
ABS_MT_SLOT          0        | ABS_MT_SLOT          0        | ABS_MT_SLOT          0
ABS_MT_POSITION_X    4000     | ABS_MT_POSITION_X    2000     | ABS_MT_POSITION_X    2000
ABS_MT_POSITION_Y    4000     | ABS_MT_POSITION_Y    2000     | ABS_MT_POSITION_Y    2000
ABS_MT_PRESSURE      78       | ABS_MT_PRESSURE      78       | ABS_MT_PRESSURE      78
ABS_MT_SLOT          1        | ABS_MT_SLOT          2        | ABS_MT_SLOT          2
ABS_MT_TRACKING_ID   -1       | ABS_MT_TRACKING_ID   55       | ABS_MT_TRACKING_ID   55
                              | ABS_MT_POSITION_X    4000     | ABS_MT_POSITION_X    4000
                              | ABS_MT_POSITION_Y    4000     | ABS_MT_POSITION_Y    4000
                              | ABS_MT_PRESSURE      72       | ABS_MT_PRESSURE      72
                              | ABS_MT_SLOT          1        |
                              | ABS_MT_TRACKING_ID   -1       |
ABS_X                4000     | ABS_X                2000     | ABS_X                2000
ABS_Y                4000     | ABS_Y                2000     | ABS_Y                2000
ABS_PRESSURE         78       | ABS_PRESSURE         78       | ABS_PRESSURE         78
BTN_TOOL_DOUBLETAP   0        | BTN_TOOL_DOUBLETAP   0        | BTN_TOOL_DOUBLETAP   0
BTN_TOOL_TRIPLETAP   1        | BTN_TOOL_TRIPLETAP   1        | BTN_TOOL_TRIPLETAP   1
--- SYN_REPORT (0) ---------- |                               |
ABS_MT_SLOT          0        |                               |
ABS_MT_POSITION_X    4000     |                               |
ABS_MT_POSITION_Y    4000     |                               |
ABS_MT_PRESSURE      78       |                               |
ABS_MT_SLOT          1        |                               |
ABS_MT_TRACKING_ID   55       |                               |
ABS_MT_POSITION_X    2000     |                               |
ABS_MT_POSITION_Y    2000     |                               |
ABS_MT_PRESSURE      72       |                               |
ABS_X                4000     |                               |
ABS_Y                4000     |                               |
ABS_PRESSURE         78       |                               |
--- SYN_REPORT (0) ---------- | --- SYN_REPORT (0) ---------- | --- SYN_REPORT (0) ----------
                              |                               |
...                           | ...                           | ...
                              |                               |
------------------------------------  3 fingers release ------------------------------------
                              |                               |
                              |                               |
--- SYN_REPORT (0) ---------- | --- SYN_REPORT (0) ---------- | --- SYN_REPORT (0) ----------
                              |                               | ABS_MT_SLOT          2
                              |                               | ABS_MT_POSITION_X    4000
                              |                               | ABS_MT_POSITION_Y    4000
                              |                               | ABS_MT_PRESSURE      45
ABS_MT_SLOT          0        | ABS_MT_SLOT          0        | ABS_MT_SLOT          0
ABS_MT_TRACKING_ID   -1       | ABS_MT_TRACKING_ID   -1       | ABS_MT_TRACKING_ID   -1
ABS_MT_SLOT          1        | ABS_MT_SLOT          2        | ABS_MT_SLOT          1
ABS_MT_TRACKING_ID   -1       | ABS_MT_TRACKING_ID   -1       | ABS_MT_TRACKING_ID   -1
                              |                               | ABS_X                4000
                              |                               | ABS_Y                4000
BTN_TOUCH            0        | BTN_TOUCH            0        | ABS_PRESSURE         45
ABS_PRESSURE         0        | ABS_PRESSURE         0        | BTN_TOOL_FINGER      1
BTN_TOOL_TRIPLETAP   0        | BTN_TOOL_TRIPLETAP   0        | BTN_TOOL_TRIPLETAP   0
--- SYN_REPORT (0) ---------- | --- SYN_REPORT (0) ---------- | --- SYN_REPORT (0) ----------
                              |                               | ABS_MT_SLOT          2
                              |                               | ABS_MT_TRACKING_ID   -1
                              |                               | BTN_TOUCH            0
                              |                               | ABS_PRESSURE         0
                              |                               | BTN_TOOL_FINGER      0
                              |                               | --- SYN_REPORT (0) ----------


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