altos/lpc: Take advantage of USART TX fifo

The USART has a 16-byte TX fifo; keep rough track of how full it is to
avoid waiting for an interrupt after every TX byte.

Signed-off-by: Keith Packard <keithp@keithp.com>

diff --git a/src/lpc/ao_serial_lpc.c b/src/lpc/ao_serial_lpc.c
index 4ecaa175f0cc69f6da06bab4330bed758ee5d3d3..c04246990cf41652bf3788a6e3ec0352d0df15d5 100644 (file)
--- a/src/lpc/ao_serial_lpc.c
+++ b/src/lpc/ao_serial_lpc.c
@@ -20,7 +20,10 @@
 
 struct ao_fifo ao_usart_rx_fifo;
 struct ao_fifo ao_usart_tx_fifo;
-uint8_t                ao_usart_tx_started;
+uint8_t                ao_usart_tx_avail;
+uint8_t                ao_usart_tx_avail_min;
+
+#define LPC_USART_TX_FIFO_SIZE 16
 
 void
 ao_debug_out(char c)
@@ -35,9 +38,10 @@ ao_debug_out(char c)
 static void
 _ao_serial_tx_start(void)
 {
-       if (!ao_fifo_empty(ao_usart_tx_fifo) & !ao_usart_tx_started)
-       {
-               ao_usart_tx_started = 1;
+       if (!ao_fifo_empty(ao_usart_tx_fifo) && ao_usart_tx_avail) {
+               ao_usart_tx_avail--;
+               if (ao_usart_tx_avail < ao_usart_tx_avail_min)
+                       ao_usart_tx_avail_min = ao_usart_tx_avail;
                ao_fifo_remove(ao_usart_tx_fifo, lpc_usart.rbr_thr);
        }
 }
@@ -56,7 +60,7 @@ lpc_usart_isr(void)
                        ao_wakeup(&ao_stdin_ready);
        }
        if (lpc_usart.lsr & (1 << LPC_USART_LSR_THRE)) {
-               ao_usart_tx_started = 0;
+               ao_usart_tx_avail = LPC_USART_TX_FIFO_SIZE;
                _ao_serial_tx_start();
                ao_wakeup(&ao_usart_tx_fifo);
        }
@@ -162,6 +166,9 @@ ao_serial_init(void)
                         (1 << LPC_USART_FCR_TXFIFORES) |
                         (LPC_USART_FCR_RXTL_1 << LPC_USART_FCR_RXTL));
 
+       ao_usart_tx_avail = LPC_USART_TX_FIFO_SIZE;
+       ao_usart_tx_avail_min = LPC_USART_TX_FIFO_SIZE;
+
        /* 8 n 1 */
        lpc_usart.lcr = ((LPC_USART_LCR_WLS_8 << LPC_USART_LCR_WLS) |
                         (LPC_USART_LCR_SBS_1 << LPC_USART_LCR_SBS) |