HAL_Linux: implement HAL_GPIO_SCRIPT support

this allows Linux boards to trigger an external script on GPIO
writes. This can be hooked up to relays with either RC options or
mavlink or mission items to trigger actions that can be scripted

libraries/AP_HAL_Linux/GPIO_Sysfs.cpp

@@ -80,10 +80,24 @@ void DigitalSource_Sysfs::toggle()
 
 void GPIO_Sysfs::init()
 {
+#ifdef HAL_GPIO_SCRIPT
+    if (!_script.thread_created) {
+        _script.thread_created = true;
+        if (!hal.scheduler->thread_create(FUNCTOR_BIND_MEMBER(&GPIO_Sysfs::_gpio_script_thread, void),
+                                          "GPIO_Script", 4096, AP_HAL::Scheduler::PRIORITY_IO, -1)) {
+            AP_HAL::panic("Unable to create GPIO_Script thread");
+        }
+    }
+#endif
 }
 
 void GPIO_Sysfs::pinMode(uint8_t vpin, uint8_t output)
 {
+#ifdef HAL_GPIO_SCRIPT
+    if (vpin >= n_pins && output) {
+        return;
+    }
+#endif
     assert_vpin(vpin, n_pins);
 
     _export_pin(vpin);
@@ -146,6 +160,12 @@ error:
 
 void GPIO_Sysfs::write(uint8_t vpin, uint8_t value)
 {
+#ifdef HAL_GPIO_SCRIPT
+    if (vpin >= n_pins) {
+        _gpio_script_write(vpin, value);
+        return;
+    }
+#endif
     assert_vpin(vpin, n_pins);
 
     const unsigned pin = pin_table[vpin];
@@ -195,6 +215,11 @@ bool GPIO_Sysfs::usb_connected(void)
 
 bool GPIO_Sysfs::_export_pin(uint8_t vpin)
 {
+#ifdef HAL_GPIO_SCRIPT
+    if (vpin >= n_pins) {
+        return false;
+    }
+#endif
     assert_vpin(vpin, n_pins, false);
 
     const unsigned int pin = pin_table[vpin];
@@ -231,3 +256,39 @@ fail_snprintf:
     hal.console->printf("GPIO_Sysfs: Unable to export pin %u.\n", pin);
     return false;
 }
+
+#ifdef HAL_GPIO_SCRIPT
+/*
+  support using an external script triggered by a write to a GPIO
+  value. This is called whenever a GPIO request is made that is for an
+  unknown pin value. The script is called by a separate thread, and
+  only one script can be run at a time. This prevents the scripts
+  using too many resources
+ */
+void GPIO_Sysfs::_gpio_script_write(uint8_t vpin, uint8_t value)
+{
+    pin_value_t pv;
+    pv.pin = vpin;
+    pv.value = value;
+    _script.pending.push(pv);
+
+}
+
+/*
+  thread for running GPIO scripts
+ */
+void GPIO_Sysfs::_gpio_script_thread(void)
+{
+    while (true) {
+        // don't run more than 20/sec
+        hal.scheduler->delay(50);
+        pin_value_t pv;
+        if (_script.pending.pop(pv)) {
+            char cmd[100];
+            snprintf(cmd, sizeof(cmd)-1, "/bin/sh %s %u %u", HAL_GPIO_SCRIPT, pv.pin, pv.value);
+            hal.console->printf("Running: %s\n", cmd);
+            system(cmd);
+        }
+    }
+}
+#endif // HAL_GPIO_SCRIPT

libraries/AP_HAL_Linux/GPIO_Sysfs.h

@@ -2,6 +2,7 @@
 
 #include "AP_HAL_Linux.h"
 #include <AP_HAL/AP_HAL.h>
+#include <AP_HAL/utility/RingBuffer.h>
 
 #include "GPIO.h"
 
@@ -67,6 +68,31 @@ protected:
      * Note: the pin is ignored if already exported.
      */
     static bool _export_pin(uint8_t vpin);
+
+#ifdef HAL_GPIO_SCRIPT
+    /*
+      support for calling external scripts based on GPIO writes
+     */
+    void _gpio_script_write(uint8_t vpin, uint8_t value);
+
+    /*
+      thread to run scripts
+     */
+    void _gpio_script_thread(void);
+
+    /*
+      control structures for _gpio_script_write
+     */
+    typedef struct {
+        uint8_t pin;
+        uint8_t value;
+    } pin_value_t;
+
+    struct {
+        bool thread_created;
+        ObjectBuffer<pin_value_t> pending{10};
+    } _script;
+#endif // HAL_GPIO_SCRIPT
 };
 
 }