linux: Add support for analog IIO devices

Add support for reading analog values via the standard Linux IIO
interface.  This can be used on Replicape boards to sample analog
input pins.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/linux/analog.c

@@ -0,0 +1,61 @@
+// Read analog values from Linux IIO device
+//
+// Copyright (C) 2017  Kevin O'Connor <kevin@koconnor.net>
+//
+// This file may be distributed under the terms of the GNU GPLv3 license.
+
+#include <fcntl.h> // open
+#include <stdio.h> // snprintf
+#include <stdlib.h> // atoi
+#include <unistd.h> // read
+#include "command.h" // shutdown
+#include "gpio.h" // gpio_adc_setup
+#include "internal.h" // report_errno
+#include "sched.h" // sched_shutdown
+
+DECL_CONSTANT(ADC_MAX, 4095); // Assume 12bit adc
+
+#define IIO_PATH "/sys/bus/iio/devices/iio:device0/in_voltage%d_raw"
+
+struct gpio_adc
+gpio_adc_setup(uint8_t pin)
+{
+    char fname[256];
+    snprintf(fname, sizeof(fname), IIO_PATH, pin);
+
+    int fd = open(fname, O_RDONLY|O_CLOEXEC);
+    if (fd < 0) {
+        report_errno("analog open", fd);
+        shutdown("Unable to open adc path");
+    }
+    int ret = set_non_blocking(fd);
+    if (ret < 0) {
+        report_errno("analog set_non_blocking", ret);
+        shutdown("Unable to set non blocking on adc path");
+    }
+    return (struct gpio_adc){ .fd = fd };
+}
+
+uint32_t
+gpio_adc_sample(struct gpio_adc g)
+{
+    return 0;
+}
+
+uint16_t
+gpio_adc_read(struct gpio_adc g)
+{
+    char buf[64];
+    int ret = pread(g.fd, buf, sizeof(buf)-1, 0);
+    if (ret <= 0) {
+        report_errno("analog read", ret);
+        shutdown("Error on analog read");
+    }
+    buf[ret] = '\0';
+    return atoi(buf);
+}
+
+void
+gpio_adc_cancel_sample(struct gpio_adc g)
+{
+}