vfs: add UART device

This should be integrated with UART driver once everything is merged

components/vfs/vfs_uart.c

@@ -0,0 +1,95 @@
+#include <string.h>
+#include "esp_vfs.h"
+#include "esp_attr.h"
+#include "sys/errno.h"
+#include "sys/lock.h"
+#include "soc/uart_struct.h"
+
+static uart_dev_t* s_uarts[3] = {&UART0, &UART1, &UART2};
+
+static int IRAM_ATTR uart_open(const char * path, int flags, int mode)
+{
+    // this is fairly primitive, we should check if file is opened read only,
+    // and error out if write is requested
+    if (strcmp(path, "/0") == 0) {
+        return 0;
+    } else if (strcmp(path, "/1") == 0) {
+        return 1;
+    } else if (strcmp(path, "/2") == 0) {
+        return 2;
+    }
+    errno = ENOENT;
+    return -1;
+}
+
+static void IRAM_ATTR uart_tx_char(uart_dev_t* uart, int c)
+{
+    while (uart->status.txfifo_cnt >= 127) {
+        ;
+    }
+    uart->fifo.rw_byte = c;
+}
+
+
+static size_t IRAM_ATTR uart_write(int fd, const void * data, size_t size)
+{
+    assert(fd >=0 && fd < 3);
+    const char *data_c = (const char *)data;
+    uart_dev_t* uart = s_uarts[fd];
+    static _lock_t stdout_lock; /* lazily initialised */
+    /* Even though newlib does stream locking on stdout, we need
+       a dedicated stdout UART lock...
+
+       This is because each task has its own _reent structure with
+       unique FILEs for stdin/stdout/stderr, so these are
+       per-thread (lazily initialised by __sinit the first time a
+       stdio function is used, see findfp.c:235.
+
+       It seems like overkill to allocate a FILE-per-task and lock
+       a thread-local stream, but I see no easy way to fix this
+       (pre-__sinit_, tasks have "fake" FILEs ie __sf_fake_stdout
+       which aren't fully valid.)
+    */
+    _lock_acquire_recursive(&stdout_lock);
+    for (size_t i = 0; i < size; i++) {
+#if CONFIG_NEWLIB_STDOUT_ADDCR
+        if (data_c[i]=='\n') {
+            uart_tx_char(uart, '\r');
+        }
+#endif
+        uart_tx_char(uart, data_c[i]);
+    }
+    _lock_release_recursive(&stdout_lock);
+    return size;
+}
+
+static int IRAM_ATTR uart_fstat(int fd, struct stat * st)
+{
+    assert(fd >=0 && fd < 3);
+    st->st_mode = S_IFCHR;
+    return 0;
+}
+
+static int IRAM_ATTR uart_close(int fd)
+{
+    assert(fd >=0 && fd < 3);
+    return 0;
+}
+
+void esp_vfs_dev_uart_register()
+{
+    esp_vfs_t vfs = {
+        .fd_offset = 0,
+        .flags = ESP_VFS_FLAG_DEFAULT,
+        .write = &uart_write,
+        .open = &uart_open,
+        .fstat = &uart_fstat,
+        .close = &uart_close,
+        .read = NULL, // TODO: implement reading from UART
+        .stat = NULL,
+        .link = NULL,
+        .unlink = NULL,
+        .rename = NULL
+    };
+    ESP_ERROR_CHECK(esp_vfs_register("/dev/uart", &vfs, NULL));
+}