[PATCH v2 2/2] xen/ns16550: Fix ISR lockup on Designware 8250 (H5)

[Awais Masood <awais.masood@vadion.com>]

On Allwinner H5 (Orange Pi PC2) 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 H5 (and H3), 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 is
added within the interrupt handler to clear the 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). Earlier this issue was seen on H3
as well and the same fix should help.

Signed-off-by: Awais Masood <awais.masood@vadion.com>

---
Changes since v1:
 * Common quirk fix code moved to a helper function
 * Patch description improved with earlier commit link
---
 xen/drivers/char/ns16550.c | 27 ++++++++++++++++++---------
 1 file changed, 18 insertions(+), 9 deletions(-)

diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index 6ab5ec3..44ed4ec 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -505,6 +505,19 @@ static int ns16550_ioport_invalid(struct ns16550 *uart)
     return ns_read_reg(uart, UART_IER) == 0xff;
 }
 
+static void ns16550_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.
+         */
+        ns_read_reg(uart, UART_USR);
+    }
+}
+
 static void ns16550_interrupt(
     int irq, void *dev_id, struct cpu_user_regs *regs)
 {
@@ -521,6 +534,9 @@ static void ns16550_interrupt(
             serial_tx_interrupt(port, regs);
         if ( lsr & UART_LSR_DR )
             serial_rx_interrupt(port, regs);
+
+        /* Handle the DesignWare 8250 'busy-detect' quirk. */
+        ns16550_handle_dw_usr_busy_quirk(uart);
     }
 }
 
@@ -623,15 +639,8 @@ 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. */
+    ns16550_handle_dw_usr_busy_quirk(uart);
 
     /* Line control and baud-rate generator. */
     ns_write_reg(uart, UART_LCR, lcr | UART_LCR_DLAB);
-- 
2.7.4


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