refactor: set /dev/gpiochip0 as default (#1018)

resolves #1017

This also adds some info to the docs that was requested in #1017

* remove unnecessary members
* make cached chip FD static

RF24.h

@@ -215,7 +215,8 @@ class RF24
      *
      * See [Related Pages](pages.html) for device specific information
      *
-     * @param _cepin The pin attached to Chip Enable on the RF module
+     * @param _cepin The pin attached to Chip Enable on the RF module.
+     * Review our [Linux general](rpi_general.md) doc for details about selecting pin numbers on Linux systems.
      * @param _cspin The pin attached to Chip Select (often labeled CSN) on the radio module.
      * - For the Arduino Due board, the [Arduino Due extended SPI feature](https://www.arduino.cc/en/Reference/DueExtendedSPI)
      * is not supported. This means that the Due's pins 4, 10, or 52 are not mandated options (can use any digital output pin) for
@@ -290,7 +291,8 @@ class RF24
      * @param spiBus A pointer or reference to an instantiated SPI bus object.
      * The `_SPI` datatype is a "wrapped" definition that will represent
      * various SPI implementations based on the specified platform.
-     * @param _cepin The pin attached to Chip Enable on the RF module
+     * @param _cepin The pin attached to Chip Enable on the RF module.
+     * Review our [Linux general](rpi_general.md) doc for details about selecting pin numbers on Linux systems.
      * @param _cspin The pin attached to Chip Select (often labeled CSN) on the radio module.
      * - For the Arduino Due board, the [Arduino Due extended SPI feature](https://www.arduino.cc/en/Reference/DueExtendedSPI)
      * is not supported. This means that the Due's pins 4, 10, or 52 are not mandated options (can use any digital output pin) for the radio's CSN pin.

cspell.config.yaml

@@ -134,6 +134,7 @@ words:
   - iomanip
   - ITSYBITSY
   - Jannis
+  - Jetson
   - jscrane
   - KBPS
   - Konde
@@ -142,6 +143,7 @@ words:
   - ldiaz
   - libboost
   - libc
+  - libgpiod
   - libmaple
   - libncurses
   - librf

docs/Doxyfile

@@ -135,6 +135,12 @@ SHOW_FILES             = YES
 
 SHOW_NAMESPACES        = NO
 
+# Don't display the ugly/complicated "tree view" side menu.
+# And, restore the top (simpler) navigation bar.
+# As of doxygen v1.13.0, these default values changed from NO to YES
+GENERATE_TREEVIEW      = NO
+DISABLE_INDEX          = NO
+
 #---------------------------------------------------------------------------
 # Configuration options related to warning and progress messages
 #---------------------------------------------------------------------------

docs/linux_install.md

@@ -24,13 +24,13 @@ Using CMake: (See the [instructions using CMake](using_cmake.md) for more inform
    ./install.sh
    ```
 
-   @warning 
+   @warning
    `SPIDEV` is now always selected as the default driver because
    all other Linux drivers are being removed in the future.
    See [RF24 issue #971](https://github.com/nRF24/RF24/issues/971) for rationale.
 
    It will also ask to install a python package named [pyRF24](https://github.com/nRF24/pyRF24).
-   This is not the same as the traditionally provided python wrappers as the pyRF24 package can be
+   This is not the same as the traditionally provided python wrappers because the pyRF24 package can be
    used independent of the C++ installed libraries. For more information on this newer python
    package, please check out [the pyRF24 documentation](https://nrf24.github.io/pyRF24/).
 4. Try an example from one of the libraries
@@ -76,7 +76,7 @@ See the [instructions using CMake](using_cmake.md) for more information and opti
    ./install.sh
    ```
 
-   @warning 
+   @warning
    `SPIDEV` is now always selected as the default driver because
    all other Linux drivers are being removed in the future.
    See [RF24 issue #971](https://github.com/nRF24/RF24/issues/971) for rationale.
@@ -115,7 +115,7 @@ See the [instructions using CMake](using_cmake.md) for more information and opti
    ```
    It automatically detects device and build environment.
 
-   @warning 
+   @warning
    `SPIDEV` is now always selected as the default driver because
    all other Linux drivers are being removed in the future.
    See [RF24 issue #971](https://github.com/nRF24/RF24/issues/971) for rationale.

docs/rpi_general.md

@@ -49,16 +49,43 @@ for proper constructor to address the correct spi device at /dev/spidev\<a\>.\<b
 
 Choose any GPIO output pin for radio CE pin.
 
+> [!WARNING]
+> The pin numbers may be different for certain systems.
+> In this library's examples, we use the BCM pin numbers for BroadCom chips such as
+> those used on all Raspberry Pi models.
+>
+> For Raspberry Pi clones, such as Orange Pi or Banana Pi, defer to the pin numbers described
+> in their corresponding documentation/manual.
+>
+> Hint: If libgpiod is installed (not required for this library),
+> then you can use the included
+> [CLI tools shipped with libgpiod](https://libgpiod.readthedocs.io/en/latest/gpio_tools.html)
+> to gleam system-specific details.
+
+### Linux kernel (SPIDEV driver) and the GPIO chip number
+
+This RF24 library, as of v1.4.9, uses the Linux kernel's Character Device API to interface
+with GPIO pins (AKA "lines" in kernel docs). Previous versions used the deprecated "sys fs" interface.
+
+By default, this library attempts to use pins exposed via `/dev/gpiochip0`.
+Some systems have multiple public-facing GPIO chips integrated (ie nVidia Jetson series).
+If your system exposes the desired GPIO pins via a different chip (ie `/dev/gpiochip4`), then the
+`RF24_LINUX_GPIO_CHIP` can be set to the correct value when compiling the library.
+
+```shell
+cmake .. -DRF24_LINUX_GPIO_CHIP="/dev/gpiochip4"
+```
+
 ### General
 
 ```cpp
-RF24 radio(22,0);
+RF24 radio(22, 0);
 ```
 
 ### MRAA Constructor
 
 ```cpp
-RF24 radio(15,0);
+RF24 radio(15, 0);
 ```
 
 See [the MRAA documentation for Raspberry Pi support](http://iotdk.intel.com/docs/master/mraa/rasppi.html)

utility/RPi/RF24_arch_config.h

@@ -29,10 +29,11 @@ typedef uint8_t rf24_gpio_pin_t;
 
 #ifndef RF24_LINUX_GPIO_CHIP
     /**
-     * The default GPIO chip to use.  Defaults to `/dev/gpiochip4` (for RPi5).
-     * Falls back to `/dev/gpiochip0` if this value is somehow incorrect.
+     * The default GPIO chip to use.
+     * Defaults to `/dev/gpiochip0`.
+     * Define this when compiling if this value is somehow incorrect.
      */
-    #define RF24_LINUX_GPIO_CHIP "/dev/gpiochip4"
+    #define RF24_LINUX_GPIO_CHIP "/dev/gpiochip0"
 #endif
 
 #define PSTR(x)  (x)

utility/SPIDEV/gpio.cpp

@@ -21,20 +21,22 @@ std::map<rf24_gpio_pin_t, gpio_fd> cachedPins;
 struct gpio_v2_line_request request;
 struct gpio_v2_line_values data;
 
+// initialize static member.
+int GPIOChipCache::fd = -1;
+
 void GPIOChipCache::openDevice()
 {
     if (fd < 0) {
-        fd = open(chip, O_RDONLY);
+        fd = open(RF24_LINUX_GPIO_CHIP, O_RDONLY);
         if (fd < 0) {
             std::string msg = "Can't open device ";
-            msg += chip;
+            msg += RF24_LINUX_GPIO_CHIP;
             msg += "; ";
             msg += strerror(errno);
             throw GPIOException(msg);
             return;
         }
     }
-    chipInitialized = true;
 }
 
 void GPIOChipCache::closeDevice()
@@ -75,31 +77,21 @@ GPIO::~GPIO()
 
 void GPIO::open(rf24_gpio_pin_t port, int DDR)
 {
-    try {
-        gpioCache.openDevice();
-    }
-    catch (GPIOException& exc) {
-        if (gpioCache.chipInitialized) {
-            throw exc;
-            return;
-        }
-        gpioCache.chip = "/dev/gpiochip0";
-        gpioCache.openDevice();
-    }
+    gpioCache.openDevice();
 
     // get chip info
     gpiochip_info info;
     memset(&info, 0, sizeof(info));
     int ret = ioctl(gpioCache.fd, GPIO_GET_CHIPINFO_IOCTL, &info);
     if (ret < 0) {
         std::string msg = "Could not gather info about ";
-        msg += gpioCache.chip;
+        msg += RF24_LINUX_GPIO_CHIP;
         throw GPIOException(msg);
         return;
     }
 
     if (port > info.lines) {
-        std::string msg = "pin number " + std::to_string(port) + " not available for " + gpioCache.chip;
+        std::string msg = "pin number " + std::to_string(port) + " not available for " + RF24_LINUX_GPIO_CHIP;
         throw GPIOException(msg);
         return;
     }

utility/SPIDEV/gpio.h

@@ -25,7 +25,7 @@ typedef uint16_t rf24_gpio_pin_t;
      * The default GPIO chip to use.  Defaults to `/dev/gpiochip4` (for RPi5).
      * Falls back to `/dev/gpiochip0` if this value is somehow incorrect.
      */
-    #define RF24_LINUX_GPIO_CHIP "/dev/gpiochip4"
+    #define RF24_LINUX_GPIO_CHIP "/dev/gpiochip0"
 #endif
 
 /** Specific exception for GPIO errors */
@@ -42,9 +42,14 @@ class GPIOException : public std::runtime_error
 /// This struct's destructor should close any cached GPIO pin requests' file descriptors.
 struct GPIOChipCache
 {
-    const char* chip = RF24_LINUX_GPIO_CHIP;
-    int fd = -1;
-    bool chipInitialized = false;
+    /// @brief The file descriptor used to access the GPIO chip.
+    ///
+    /// This is used to open/close pins exposed by the GPIO chip specified via
+    /// `RF24_LINUX_GPIO_CHIP`.
+    ///
+    /// Because this member is static, all instances (& derivative instances) of this
+    /// struct use the same file descriptor.
+    static int fd;
 
     /// Open the File Descriptor for the GPIO chip
     void openDevice();

utility/SPIDEV/interrupt.cpp

@@ -65,31 +65,21 @@ int attachInterrupt(rf24_gpio_pin_t pin, int mode, void (*function)(void))
     detachInterrupt(pin);
     GPIO::close(pin);
 
-    try {
-        irqChipCache.openDevice();
-    }
-    catch (GPIOException& exc) {
-        if (irqChipCache.chipInitialized) {
-            throw exc;
-            return 0;
-        }
-        irqChipCache.chip = "/dev/gpiochip0";
-        irqChipCache.openDevice();
-    }
+    irqChipCache.openDevice();
 
     // get chip info
     gpiochip_info info;
     memset(&info, 0, sizeof(info));
     int ret = ioctl(irqChipCache.fd, GPIO_GET_CHIPINFO_IOCTL, &info);
     if (ret < 0) {
         std::string msg = "[attachInterrupt] Could not gather info about ";
-        msg += irqChipCache.chip;
+        msg += RF24_LINUX_GPIO_CHIP;
         throw IRQException(msg);
         return 0;
     }
 
     if (pin > info.lines) {
-        std::string msg = "[attachInterrupt] pin " + std::to_string(pin) + " is not available on " + irqChipCache.chip;
+        std::string msg = "[attachInterrupt] pin " + std::to_string(pin) + " is not available on " + RF24_LINUX_GPIO_CHIP;
         throw IRQException(msg);
         return 0;
     }