A single key on keyboard produces extra keypresses for each simultaneously pressed key

Alright, I managed to put together a hack that solves the issue. I'll write it here in case someone stumbles with the same issue.

First off, if you're not interested in tweaking the kernel source, you might have another option: http://kbd-mangler.sourceforge.net/ - I did not test it, but the description looks rather promising. It allows you to tweak input before it's passed to the system.

My solution was to edit the file drivers/hid/hid-input.c. In the beginning of the file I added three new variable definitions;

static bool CODE43TRUE = 0; // If true, code43 has been pressed static bool CODEXXTRUE = 0; // If true, any other key has been pressed static int CODESKIP = 0; // Counter for skipping extra code43 events 

Find the function

void hidinput_hid_event 

At the bottom of this function is

input_event(input, usage->type, usage->code, value); 

Input is the controller, type refers to event type (1 is keypress.. 2 is mouse motion?), code is the keycode and value is either 0 for depressed and 1 for pressed.

At any keypress the HID system loops 4 times through all keyboard keys. Why does it do it 4 times I do not know, but 4 corresponds with the amount of extra keypresses I got with my problematic key. On the first loop the pressed key has value 0, on second value 1, and on third and fourth again value 0.

The solution was to modify this function so that it would not allow the problematic key to be pressed again when other keys are pressed OR within 4 loops of the original keypress. This was achieved by the following code (Did I mention I haven't coded C for at least a decade? Sorry)

/* report the usage code as scancode if the key status has changed */ if (usage->type == EV_KEY && !!test_bit(usage->code, input->key) != value) input_event(input, EV_MSC, MSC_SCAN, usage->hid); // NEW CODE STARTS HERE if (usage->type == 1 && value == 1) // Keypress ahead { if (usage->code == 43) { // Keypress is code 43 if (CODE43TRUE == 0) { // Key not yet pressed CODE43TRUE = 1; printk(KERN_INFO "CODE43 SET TRUE\n"); } else { // Key already pressed, so force value 1 printk(KERN_INFO "CODE43 ALREADY TRUE SET VALUE 1\n"); value = 0; } } else { // Some other key pressed, set XX true CODEXXTRUE = 1; printk(KERN_INFO "CODEXX SET TRUE\n"); } printk(KERN_INFO "Keypress type:%u code:%u value%d\n", (unsigned int) usage->type, (unsigned int) usage->code, (int) value); } if (usage->type == 1 && value == 0) { // Non-pressed key ahead if (usage->code == 43) { // If its a 43 printk(KERN_INFO "43 call..\n"); if (CODE43TRUE == 1) { // And 43 is fake pressed still if (CODEXXTRUE == 1 || CODESKIP < 4) { // If other buttons are pressed OR we are less than 5 ticks into the press.. printk(KERN_INFO "FAKE PRESS 43. CODESKIP %d\n",CODESKIP); value = 0; CODESKIP ++; } else { // No other buttons pressed and over five ticks have passed printk(KERN_INFO "43 RELEASED\n"); CODE43TRUE = 0; CODESKIP = 0; } } // Reset the CODEXXTRUE (next time we get info about 43, we have looped through all the other keys so we know if something is pressed) CODEXXTRUE = 0; } } // NEW CODE ENDS HERE input_event(input, usage->type, usage->code, value); 

If you're implementing this you might want to remove the printk statements once you've verified it's working as intended. They're just there to help debugging.