[PATCH v2 1/2] xen/ns16550: detect and preserve UART_IER_UUE

[Zhang Zheng <zhangzheng@xxxxxxxxxxx>]

XScale-compatible UARTs require UART_IER_UUE to remain set.  Probe the
bit during setup and preserve it when enabling receive interrupts, so
the xscale/PXA UART remains operational after ns16550 takes over.

Signed-off-by: Zhang Zheng <zhangzheng@xxxxxxxxxxx>
---
 xen/drivers/char/ns16550.c  | 12 +++++++++++-
 xen/include/xen/8250-uart.h |  1 +
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index cdda28879e..c174465059 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -64,6 +64,7 @@ static struct ns16550 {
     bool intr_works;
     bool force_polling;
     bool dw_usr_bsy;
+    bool xscale;
 #ifdef NS16550_PCI
     /* PCI card parameters. */
     bool pb_bdf_enable;     /* if =1, pb-bdf effective, port behind bridge */
@@ -352,6 +353,14 @@ static void ns16550_setup_preirq(struct ns16550 *uart)
     /* No interrupts. */
     ns_write_reg(uart, UART_IER, 0);
 
+    uart->xscale = false;
+    if ( !(ns_read_reg(uart, UART_IER) & UART_IER_UUE) )
+    {
+        ns_write_reg(uart, UART_IER, UART_IER_UUE);
+        if ( ns_read_reg(uart, UART_IER) & UART_IER_UUE )
+            uart->xscale = true;
+    }
+
     /* Handle the DesignWare 8250 'busy-detect' quirk. */
     handle_dw_usr_busy_quirk(uart);
 
@@ -446,7 +455,8 @@ static void ns16550_setup_postirq(struct ns16550 *uart)
                      UART_MCR, UART_MCR_OUT2 | UART_MCR_DTR | UART_MCR_RTS);
 
         /* Enable receive interrupts. */
-        ns_write_reg(uart, UART_IER, UART_IER_ERDAI);
+        ns_write_reg(uart, UART_IER,
+                     UART_IER_ERDAI | (uart->xscale ? UART_IER_UUE : 0));
     }
 
     if ( uart->irq >= 0 )
diff --git a/xen/include/xen/8250-uart.h b/xen/include/xen/8250-uart.h
index d13352940c..358d264698 100644
--- a/xen/include/xen/8250-uart.h
+++ b/xen/include/xen/8250-uart.h
@@ -42,6 +42,7 @@
 #define UART_IER_ETHREI   0x02    /* tx reg. empty        */
 #define UART_IER_ELSI     0x04    /* rx line status       */
 #define UART_IER_EMSI     0x08    /* MODEM status         */
+#define UART_IER_UUE      0x40    /* UART unit enable (XScale/PXA) */
 
 /* Interrupt Identification Register */
 #define UART_IIR_NOINT    0x01    /* no interrupt pending */
-- 
2.54.0