g: make uart run faster

The EC queue library wrappers are very heavy, let's bypass them and
use direct queue access where performance matters the most, in the
UART RX driver used by USB streams.

BUG=b:38448364
BRANCH=cr50, cr50-mp
TEST=with the rest of the patches in place observed a much more
      reliable streaming of two consoles (ec and ap), both pumping
      chargen streams into their respective UARTs on an Octopus
      device.

Change-Id: I45dc8f1c0841b43e17ef67e96820669053fba831
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1672208
Reviewed-by: Namyoon Woo <namyoon@chromium.org>

1 files changed, 17 insertions, 14 deletions

diff --git a/chip/g/usart.c b/chip/g/usart.c
index e3e83dd1f2..0a08d261b8 100644
--- a/chip/g/usart.c
+++ b/chip/g/usart.c
@@ -145,26 +145,29 @@ void get_data_from_usb(struct usart_config const *config)
 
 void send_data_to_usb(struct usart_config const *config)
 {
-	/*
-	 * UART RX FIFO is 32 bytes in size, let's have little extra room so
-	 * that we could catch up if we are draining the FIFO while the chip
-	 * keeps receiving.
-	 */
-	uint8_t buffer[50];
-	uint32_t i;
-	uint32_t room;
 	struct queue const *uart_in = config->producer.queue;
 	int uart = config->uart;
+	size_t count;
+	size_t q_room;
+	size_t tail;
+	size_t mask;
 
+	q_room = queue_space(uart_in);
 
-	i = 0;
-	room = MIN(sizeof(buffer), queue_space(uart_in));
+	if (!q_room)
+		return;
 
-	while ((i < room) && uartn_rx_available(uart))
-		buffer[i++] = uartn_read_char(uart);
+	mask = uart_in->buffer_units_mask;
+	tail = uart_in->state->tail & mask;
+	count = 0;
 
-	if (i)
-		QUEUE_ADD_UNITS(uart_in, buffer, i);
+	while ((count != q_room) && uartn_rx_available(uart)) {
+		uart_in->buffer[tail] = uartn_read_char(uart);
+		tail = (tail + 1) & mask;
+		count++;
+	}
+	if (count)
+		queue_advance_tail(uart_in, count);
 }
 
 static void uart_read(struct producer const *producer, size_t count)