Bug 92766

Summary: xorg.conf ignores ConstantDeceleration <1
Product: xorg Reporter: nikolaos.bezirgiannis <bezeria>
Component: Server/Input/CoreAssignee: Xorg Project Team <xorg-team>
Status: RESOLVED FIXED QA Contact: Xorg Project Team <xorg-team>
Severity: normal    
Priority: medium    
Version: unspecified   
Hardware: Other   
OS: Linux (All)   
Whiteboard:
i915 platform: i915 features:
Attachments:
Description Flags
proposed patch none

Description nikolaos.bezirgiannis 2015-11-01 16:39:41 UTC 
I use this in my xorg.conf:

Section "InputClass"
	Identifier "All Mice"
	MatchIsPointer "yes"
	Option "AccelerationProfile" "-1"
	Option "ConstantDeceleration" "0.5"  # This is smaller <1
EndSection

If I set ConstantDeceleration as a float < 1 , on next start, X sets it to 1 instead:

> xinput --list-props 14
...
Device Accel Constant Deceleration (265):	1.000000

If I do instead this:

> xinput --set-prop 14 'Device Accel Constant Deceleration' 0.5
> xinput --list-props 14
...
Device Accel Constant Deceleration (265):	0.500000

This works just fine. Also any value >1 will work with xorg.conf
Comment 1 nikolaos.bezirgiannis 2015-11-01 16:40:25 UTC 
These has been tried with multiple mice, so I think it is not related to a driver or device.
Comment 2 Alan Coopersmith 2015-11-01 16:53:10 UTC 
git grep shows ConstantDeceleration is handled in the core xserver input
code, not any of the drivers under Input/*, so moving to that component.
Comment 3 ebiggers3 2016-06-09 03:18:57 UTC 
I encountered this bug as well.  It occurs if the "ConstantDeceleration" option is specified in xorg.conf.  It does not occur if the "Device Accel Constant Deceleration" is set using xinput.  The source of the bug is in ProcessVelocityConfiguration() in xf86Xinput.c, where the value of "ConstantDeceleration" is only used if it is > 1.0.  I verified that the value is recognized correctly after changing the conditional to != 1.0.

Based on the code, "AdaptiveDeceleration" is also subject to the same bug.

Note that xorg.conf(5) explicitly says that a value of ConstantDeceleration between 0 and 1 is allowed and will "speed up the pointer".  This is the expected behavior, in my opinion.
Comment 4 ebiggers3 2016-06-09 03:31:09 UTC 
Created attachment 124409 [details] [review]
proposed patch

Attached a proposed patch. I also sent it to the mailing list.
Comment 5 ebiggers3 2016-06-18 20:22:52 UTC 
A slightly modified version of my patch was committed (da9fec4eddd554b4b709ba58b4436aef5a76cd51).  This solves the problem.  Closing this bug.
Comment 6 Vasili Pupkin 2016-09-02 19:04:25 UTC 
What is the point in discussing bugs and making patches if bugfixes aren't included in 1.18? The bugfix will finally land in popular distributives like Ubuntu not earlier than the beginning of 2018.

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