gpio_api: API for GPIO controllers

- Somewhat similar to chardev API, but for GPIO pins
- For controllers: Use gpio_pins_out() to signal about pins state change; Hangle gpio->pins_in() & gpio->pins_read() for setting input pins or reading output pins
- For devices: Use gpio_pins_in() to write pins to controller, gpio_pins_read() to read output pins; Handle (optionally) gpio->pins_out() to be notified when output pins change
- Request for comments

src/devices/gpio_api.h

@@ -0,0 +1,68 @@
+/*
+gpio_api.h - General-Purpose IO API
+Copyright (C) 2024  LekKit <github.com/LekKit>
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+
+#ifndef RVVM_GPIO_API_H
+#define RVVM_GPIO_API_H
+
+typedef struct rvvm_gpio_dev rvvm_gpio_dev_t;
+
+struct rvvm_gpio_dev {
+    // IO Dev -> GPIO Dev calls
+    bool (*pins_out)(rvvm_gpio_dev_t* dev, size_t off, uint32_t pins);
+
+    // GPIO Dev -> IO Dev calls
+    bool     (*pins_in)(rvvm_gpio_dev_t* dev, size_t off, uint32_t pins);
+    uint32_t (*pins_read)(rvvm_gpio_dev_t* dev, size_t off);
+
+    // Common RVVM API features
+    void (*update)(rvvm_gpio_dev_t* dev);
+    void (*remove)(rvvm_gpio_dev_t* dev);
+
+    void* data;
+    void* io_dev;
+};
+
+static inline bool gpio_pins_out(rvvm_gpio_dev_t* dev, size_t off, uint32_t pins)
+{
+    if (dev) return dev->pins_out(dev, off, pins);
+    return false;
+}
+
+static inline bool gpio_pins_in(rvvm_gpio_dev_t* dev, size_t off, uint32_t pins)
+{
+    if (dev) return dev->pins_in(dev, off, pins);
+    return false;
+}
+
+static inline uint32_t gpio_pins_read(rvvm_gpio_dev_t* dev, size_t off)
+{
+    if (dev) return dev->pins_read(dev, off);
+    return 0;
+}
+
+static inline void gpio_free(rvvm_gpio_dev_t* dev)
+{
+    if (dev && dev->remove) dev->remove(dev);
+}
+
+static inline void gpio_update(rvvm_gpio_dev_t* dev)
+{
+    if (dev && dev->update) dev->update(dev);
+}
+
+#endif