[Xen-devel] [PATCH v1] xen/serial: Return actual bytes stored in TX FIFO for OMAP

[Oleksandr Tyshchenko <oleksandr.tyshchenko@xxxxxxxxxxxxxxx>]

This is intended to decrease a time spending in transmitter
while waiting for the free space in TX FIFO.
And as result to reduce the impact of hvc on the entire system
running on OMAP5/DRA7XX based platforms.

Signed-off-by: Oleksandr Tyshchenko <oleksandr.tyshchenko@xxxxxxxxxxxxxxx>
CC: Ian Campbell <ian.campbell@xxxxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxxxxx>
CC: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
---
 xen/drivers/char/omap-uart.c | 11 ++++++++++-
 xen/include/xen/8250-uart.h  |  9 +++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/xen/drivers/char/omap-uart.c b/xen/drivers/char/omap-uart.c
index d8f64ea..8c542a7 100644
--- a/xen/drivers/char/omap-uart.c
+++ b/xen/drivers/char/omap-uart.c
@@ -233,11 +233,20 @@ static int omap_uart_tx_ready(struct serial_port *port)
 {
     struct omap_uart *uart = port->uart;
     uint32_t reg;
+    uint8_t cnt;
 
     reg = omap_read(uart, UART_IER);
     omap_write(uart, UART_IER, reg | UART_IER_ETHREI);
 
-    return omap_read(uart, UART_LSR) & UART_LSR_THRE ? uart->fifo_size : 0;
+    /* Check for empty space in TX FIFO */
+    if ( omap_read(uart, UART_OMAP_SSR) & UART_OMAP_SSR_TX_FIFO_FULL_MASK )
+        return 0;
+
+    /* Check number of data bytes stored in TX FIFO */
+    cnt = omap_read(uart, UART_OMAP_TXFIFO_LVL);
+    ASSERT( cnt >= 0 && cnt <= uart->fifo_size );
+
+    return (uart->fifo_size - cnt);
 }
 
 static void omap_uart_putc(struct serial_port *port, char c)
diff --git a/xen/include/xen/8250-uart.h b/xen/include/xen/8250-uart.h
index 304b9dd..700b2d4 100644
--- a/xen/include/xen/8250-uart.h
+++ b/xen/include/xen/8250-uart.h
@@ -143,6 +143,15 @@
 /* Supplementary control register */
 #define UART_OMAP_SCR     0x10
 
+/* Supplementary status register */
+#define UART_OMAP_SSR     0x11
+
+/* Supplementary status register bitmasks */
+#define UART_OMAP_SSR_TX_FIFO_FULL_MASK (1 << 0)
+
+/* TX FIFO level register */
+#define UART_OMAP_TXFIFO_LVL   0x1A
+
 /* SCR register bitmasks */
 #define OMAP_UART_SCR_RX_TRIG_GRANU1_MASK (1 << 7)
 
-- 
1.9.1


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