zephyr/src: Increase UART input buffer to 512 bytes and reduce latency.

There are two changes here:

1. Increase the UART input bufffer to 512 bytes.  That's necessary to get
   basic REPL reliability tests working, and helps improve `mpremote`
   usage, eg copying large files.

2. Remove `uart_sem` semaphore.  This is no longer needed because
   `zephyr_getchar()` should be fully non-blocking and have as low a
   latency as possible.  `mp_hal_stdin_rx_chr()` (which calls
   `zephyr_getchar`) already uses `MICROPY_EVENT_POLL_HOOK` to get
   an efficient wait, and doing an extra wait and check for the
   semaphore in `zephyr_getchar()` just introduces unnecessary latency and
   can lead to slower input, and potentially overflowing the UART input
   buffer.

Signed-off-by: Damien George <damien@micropython.org>

1 files changed, 3 insertions, 9 deletions

diff --git a/ports/zephyr/src/zephyr_getchar.c b/ports/zephyr/src/zephyr_getchar.c
index 7660e3cc1..bf504a97c 100644
--- a/ports/zephyr/src/zephyr_getchar.c
+++ b/ports/zephyr/src/zephyr_getchar.c
@@ -23,12 +23,10 @@
 extern int mp_interrupt_char;
 void mp_sched_keyboard_interrupt(void);
 void mp_hal_signal_event(void);
-void mp_hal_wait_sem(struct k_sem *sem, uint32_t timeout_ms);
 
-static struct k_sem uart_sem;
-#define UART_BUFSIZE 256
+#define UART_BUFSIZE (512)
 static uint8_t uart_ringbuf[UART_BUFSIZE];
-static uint8_t i_get, i_put;
+static uint16_t i_get, i_put;
 
 static int console_irq_input_hook(uint8_t ch) {
     int i_next = (i_put + 1) & (UART_BUFSIZE - 1);
@@ -44,8 +42,6 @@ static int console_irq_input_hook(uint8_t ch) {
         uart_ringbuf[i_put] = ch;
         i_put = i_next;
     }
-    // printk("%x\n", ch);
-    k_sem_give(&uart_sem);
     return 1;
 }
 
@@ -55,8 +51,7 @@ int zephyr_getchar_check(void) {
 }
 
 int zephyr_getchar(void) {
-    mp_hal_wait_sem(&uart_sem, 0);
-    if (k_sem_take(&uart_sem, K_MSEC(0)) == 0) {
+    if (i_get != i_put) {
         unsigned int key = irq_lock();
         int c = (int)uart_ringbuf[i_get++];
         i_get &= UART_BUFSIZE - 1;
@@ -67,7 +62,6 @@ int zephyr_getchar(void) {
 }
 
 void zephyr_getchar_init(void) {
-    k_sem_init(&uart_sem, 0, UINT_MAX);
     uart_console_in_debug_hook_install(console_irq_input_hook);
     // All NULLs because we're interested only in the callback above
     uart_register_input(NULL, NULL, NULL);