gpio: Add mraa_get_gpiochip_base

This allows to retrieve the GPIO base using a stable device path as
found in sysfs. We cannot rely on the absolute GPIO numbers because
Linux prefers dynamic numbering. So, different probe ordering or
additional GPIO chips can ruin our day if we just use hard-coded
numbers.

This is a building block to enable upstream kernels on Galileo Gen2
and IOT2000.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

include/mraa_internal.h

@@ -179,6 +179,14 @@ uint32_t mraa_add_from_lockfile(const char* imraa_lock_file);
 mraa_result_t imraa_init();
 #endif
 
+/**
+ * helper function to find the gpiochip base corresponding to a device path
+ *
+ * @param device_path sysfs path to device
+ * @return GPIO base number or -1
+ */
+int mraa_get_gpiochip_base(const char* device_path);
+
 #ifdef __cplusplus
 }
 #endif

src/gpio/gpio.c

@@ -40,6 +40,32 @@
 #define MAX_SIZE 64
 #define POLL_TIMEOUT
 
+int mraa_get_gpiochip_base(const char* device_path)
+{
+    char *base_file;
+    char buf[1024];
+    FILE* fh;
+    int ret;
+
+    snprintf(buf, sizeof(buf) - 1, "%s/gpio/gpiochip*/base", device_path);
+    base_file = mraa_file_unglob(buf);
+    if (!base_file)
+        return -1;
+
+    fh = fopen(base_file, "r");
+    if (!fh)
+        return -1;
+
+    ret = fread(buf, 1, sizeof(buf) - 1, fh);
+    fclose(fh);
+
+    if (ret < 0)
+        return -1;
+
+    buf[ret] = 0;
+    return strtol(buf, NULL, 10);
+}
+
 static mraa_result_t
 mraa_gpio_get_valfp(mraa_gpio_context dev)
 {