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Active PS/2 multiplexing is a method for attaching up to four PS/2 pointing devices to a computer. Enabling of multiplexed mode allows commands to be directed to individual devices using routing prefixes. Multiplexed mode reports input with each byte tagged to identify its source. This method differs from one currently supported by psm(4) where so called guest device (trackpoint) is attached to special interface located on the host device (touchpad) and latter performs guest protocol conversion to special encapsulation packet format. At present time active PS/2 multiplexing is used in some models of HP laptops e.g. EliteBook 8560w, 9470m. Enabling of absolute operation mode on such touchpads is connected with following problems: 1. Touchpad's port priority is lower than trackpoint's. That blocks information queries thus prevents touchpad detection and configuration. 2. Touchpad and trackpoint have different protocol packet sizes and sync bytes. As PS/2 usage is on decline only minimal possible set of changes to support Synaptics touchpad and generic mouses is implemented. Active multiplexing mode is enabled only at probe stage to scan through attached PS/2 devices to query and configure Synaptics touchpad. After touchpad has been configured, mux is switched back to legacy (hidden multiplexing) mode to perform normal interrupt-driven input data processing. Overflow bit values rather than tags are used to separate packets produced by different devices. Switching back to legacy mode allows to avoid psm(4) and atkbd(4) rework to support 4 instances of mouse driver. Note: While in hidden multiplexing mode KBC does some editing of the packet stream. It remembers the button bits from the last packet received from each device, and replaces the button bits of every packet with the logical OR of all devices’ most recent button bits. This sort of button crosstalk results in spurious button events which are inhibitted with various tricks. E.g. trackpoint middle button events are suppressed while trackpad surface is touched and touchpad left and right button events are suppressed if corresponding trackpoint buttons are pressed. PR: 231058 Reported by: Michael Figiel <mifigiel@gmail.com> Tested by: Michael Figiel <mifigiel@gmail.com> MFC after: 2 weeks
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