From 412376e8a47184d4ee5de6e4789fba2b2330fca6 Mon Sep 17 00:00:00 2001
From: Sophie Tyalie <dev@flowerpot.me>
Date: Fri, 11 Nov 2022 15:23:30 +0100
Subject: [PATCH] kscan: Limit maximum amount of key presses to 3

When using a keyboard without diodes in the key matrix, ghosting can
become a serious issue. Quite often three keys held at the same time are
needed to produce a ghosted fourth key press when using a row, col based
keyboard matrix. In these cases it makes sense to allow only three key
presses at the same time, ignoring all of them if a fourth is pressed.

The changes made here are a simple work-of-proof how this "limit the
amount of key presses" could be done.

Note: There's no configuration options or similar to change this
behavior right now.

Signed-off-by: Sophie Tyalie <dev@flowerpot.me>
---
 app/drivers/kscan/kscan_gpio_matrix.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/app/drivers/kscan/kscan_gpio_matrix.c b/app/drivers/kscan/kscan_gpio_matrix.c
index 71fcad29..078b2ab3 100644
--- a/app/drivers/kscan/kscan_gpio_matrix.c
+++ b/app/drivers/kscan/kscan_gpio_matrix.c
@@ -225,6 +225,8 @@ static int kscan_matrix_read(const struct device *dev) {
     struct kscan_matrix_data *data = dev->data;
     const struct kscan_matrix_config *config = dev->config;
 
+    int num_active = 0;
+    int stored_states[3] = {0};
     // Scan the matrix.
     for (int o = 0; o < config->outputs.len; o++) {
         const struct gpio_dt_spec *out_gpio = &config->outputs.gpios[o];
@@ -245,8 +247,19 @@ static int kscan_matrix_read(const struct device *dev) {
             const int index = state_index_io(config, i, o);
             const bool active = gpio_pin_get_dt(in_gpio);
 
-            debounce_update(&data->matrix_state[index], active, config->debounce_scan_period_ms,
-                            &config->debounce_config);
+
+            if (active) {
+                if (num_active < sizeof(stored_states) / sizeof(stored_states[0])) {
+                    stored_states[num_active] = index;
+                }
+                num_active++;
+            } else {
+                debounce_update(&data->matrix_state[index], active,
+                                config->debounce_scan_period_ms,
+                                &config->debounce_config);
+            }
+
+
         }
 
         err = gpio_pin_set_dt(out_gpio, 0);
@@ -260,6 +273,14 @@ static int kscan_matrix_read(const struct device *dev) {
 #endif
     }
 
+    if (0 < num_active && num_active <= 3) {
+        for (int i = 0; i < num_active; i++) {
+            int index = stored_states[i];
+            debounce_update(&data->matrix_state[index], true, config->debounce_scan_period_ms,
+                            &config->debounce_config);
+        }
+    }
+
     // Process the new state.
     bool continue_scan = false;