drivers/sx126x: improve hardware detection

- split out the detection logic into a static helper function
- replace a forgotten debug `printf()` with `DEBUG()`
- add a `LOG_ERROR()` to when no device is detect to ease diagnosis of
  a failing SPI bus / incorrect wiring / broken chip
- use a bullet-proof test:
    - First set the modulation type to LoRa
    - Then get the modulation type and expect it to be LoRa

Even if the reset of the devices fails (e.g. because the reset signal
is not connected) and the modulation would be different from LoRa, this
now should reliably detect the chip.

Author: maribu

drivers/sx126x/sx126x.c

@@ -22,6 +22,7 @@
 
 #include "sx126x_netdev.h"
 
+#include "log.h"
 #include "macros/units.h"
 #include "net/lora.h"
 #include "periph/spi.h"
@@ -321,6 +322,35 @@ static void _dio1_isr(void *arg)
 }
 #endif
 
+/**
+ * @brief   Detect whether an SX126x device is actually connected
+ * @param   dev     Initialized device descriptor to use
+ * @pre     @p dev is initialized and the /CS pin has been configured
+ *
+ * This sadly is a bit more involved than one would hope, as no vendor and
+ * model codes are in the register map that can be searched for. Instead,
+ * we write the packet type (or rather modulation type) to be 100% the device
+ * must be configured to use LoRa modulation, then read this back and expect
+ * it to be LoRa. In we also check the device status to be valid, which adds
+ * one more known bit to compare against.
+ */
+static bool _sx126x_detect(sx126x_t *dev)
+{
+    sx126x_pkt_type_t pkt_type = UINT8_MAX;
+    sx126x_chip_status_t radio_status = { 0 };
+    sx126x_set_pkt_type(dev, SX126X_PKT_TYPE_LORA);
+    sx126x_get_status(dev, &radio_status);
+    sx126x_get_pkt_type(dev, &pkt_type);
+
+    if ((pkt_type == SX126X_PKT_TYPE_LORA) && (radio_status.chip_mode)) {
+        return true;
+    }
+    DEBUG("[sx126x] pkt_type = %x, radio_status.chip_mode = %u\n",
+               (unsigned)pkt_type, (unsigned)radio_status.chip_mode);
+    LOG_ERROR("[sx126x] No device detected\n");
+    return false;
+}
+
 int sx126x_init(sx126x_t *dev)
 {
     /* Setup SPI for SX126X */
@@ -355,22 +385,7 @@ int sx126x_init(sx126x_t *dev)
     /* Reset the device */
     sx126x_reset(dev);
 
-    /* Read status and packet type to verify device presence. Just reading the
-     * status can lead incorrectly detecting the chip to be present, e.g. when
-     * CIPO (a.k.a. MISO) is pulled high. The packet type after reset should be
-     * valid. The chance of it not being valid is quite good on floating SPI.
-     * If CIPO is pulled down, `radio_status.chip_mode` will be zero, if CIPO
-     * is pulled high, `pkt_type` will be `UINT8_MAX`. So in those two cases,
-     * an unconnected chip will be detected reliably.
-     */
-    sx126x_pkt_type_t pkt_type = UINT8_MAX;
-    sx126x_chip_status_t radio_status = { 0 };
-    sx126x_get_pkt_type(dev, &pkt_type);
-    sx126x_get_status(dev, &radio_status);
-    if ((pkt_type > SX126X_PKT_TYPE_LR_FHSS) || !radio_status.chip_mode) {
-        DEBUG("[sx126x] error: no device found\n");
-        printf("pkt_type = %x, radio_status.chip_mode = %u\n",
-               (unsigned)pkt_type, (unsigned)radio_status.chip_mode);
+    if (!_sx126x_detect(dev)) {
         return -ENODEV;
     }