ns16550: fix ISR lockup on Allwinner uart

This patch fixes an ISR lockup seen on Allwinner uart

On Allwinner H5, serial driver goes into an infinite loop
when interrupts are enabled. The reason is a residual
"busy detect" interrupt. Since the condition UART_IIR_NOINT
will not be true unless this interrupt is cleared, the
interrupt handler will remain locked up in this while loop.

A HW quirk fix was previously added for designware uart under
commit:
50417cd978aa54930d065ac1f139f935d14af76d

It checks for a busy condition during setup and clears the
condition by reading UART_USR register.

On Allwinner hardware, the "busy detect" condition occurs
later because an LCR write is performed during setup 'after'
this clear and if uart is busy, the "busy detect" condition
will trigger again and cause the ISR lockup.

To solve this problem, the same UART_USR read operation needs
to be performed within the interrupt handler to clear this
condition.

Linux dw 8250 driver also handles this condition within
interrupt handler
http://elixir.free-electrons.com/linux/latest/source/drivers/tty/serial/8250/8250_dw.c#L233

Tested on Orange Pi PC2 (H5). This issue is seen on H3
as well and the same fix works.

Signed-off-by: Awais Masood <awais.masood@vadion.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index 6ab5ec3def51e0f98b31dc1fa76f5002826a3c48..e0f8199f9833f44b51ec1e0f03d08fb6ff972cdb 100644 (file)
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -505,6 +505,23 @@ static int ns16550_ioport_invalid(struct ns16550 *uart)
     return ns_read_reg(uart, UART_IER) == 0xff;
 }
 
+static void handle_dw_usr_busy_quirk(struct ns16550 *uart)
+{
+    if ( uart->dw_usr_bsy &&
+         (ns_read_reg(uart, UART_IIR) & UART_IIR_BSY) == UART_IIR_BSY )
+    {
+        /* DesignWare 8250 detects if LCR is written while the UART is
+         * busy and raises a "busy detect" interrupt. Read the UART
+         * Status Register to clear this state.
+         *
+         * Allwinner/sunxi UART hardware is similar to DesignWare 8250
+         * and also contains a "busy detect" interrupt. So this quirk
+         * fix will also be used for Allwinner UART.
+         */
+        ns_read_reg(uart, UART_USR);
+    }
+}
+
 static void ns16550_interrupt(
     int irq, void *dev_id, struct cpu_user_regs *regs)
 {
@@ -521,6 +538,16 @@ static void ns16550_interrupt(
             serial_tx_interrupt(port, regs);
         if ( lsr & UART_LSR_DR )
             serial_rx_interrupt(port, regs);
+
+        /* A "busy-detect" condition is observed on Allwinner/sunxi UART
+         * after LCR is written during setup. It needs to be cleared at
+         * this point or UART_IIR_NOINT will never be set and this loop
+         * will continue forever.
+         *
+         * This state can be cleared by calling the dw_usr_busy quirk
+         * handler that resolves "busy-detect" for  DesignWare uart.
+         */
+        handle_dw_usr_busy_quirk(uart);
     }
 }
 
@@ -623,15 +650,8 @@ static void ns16550_setup_preirq(struct ns16550 *uart)
     /* No interrupts. */
     ns_write_reg(uart, UART_IER, 0);
 
-    if ( uart->dw_usr_bsy &&
-         (ns_read_reg(uart, UART_IIR) & UART_IIR_BSY) == UART_IIR_BSY )
-    {
-        /* DesignWare 8250 detects if LCR is written while the UART is
-         * busy and raises a "busy detect" interrupt. Read the UART
-         * Status Register to clear this state.
-         */
-        ns_read_reg(uart, UART_USR);
-    }
+    /* Handle the DesignWare 8250 'busy-detect' quirk. */
+    handle_dw_usr_busy_quirk(uart);
 
     /* Line control and baud-rate generator. */
     ns_write_reg(uart, UART_LCR, lcr | UART_LCR_DLAB);