[Xen-devel] [PATCH] ns16550: delay resume until dom0 ACPI has a chance to run (v2)

[Ben Guthro <benjamin.guthro@xxxxxxxxxx>]

Check for ioport access, before fully resuming operation, to avoid
spinning in __ns16550_poll when reading the LSR register returns 0xFF
on failing ioport access.

On some systems (like Lenovo T410, and some HP machines of similar vintage)
there is a SuperIO card that provides this legacy ioport on the LPC bus.

In this case, we need to wait for dom0's ACPI processing to run the proper
AML to re-initialize the chip, before we can use the card again.

This may cause a small amount of garbage to be written to the serial log
while we wait patiently for that AML to be executed.

This implementation limits the number of retries, to avoid a situation
where we keep trying over and over again, in the case of some other failure
on the ioport.

v2: Formatting, comment adjustment

Signed-Off-By: Ben Guthro <benjamin.guthro@xxxxxxxxxx>
---
 xen/drivers/char/ns16550.c |   59 +++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 58 insertions(+), 1 deletion(-)

diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index a91743e..7390874 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -45,6 +45,7 @@ static struct ns16550 {
     struct irqaction irqaction;
     /* UART with no IRQ line: periodically-polled I/O. */
     struct timer timer;
+    struct timer resume_timer;
     unsigned int timeout_ms;
     bool_t intr_works;
     /* PCI card parameters. */
@@ -123,6 +124,10 @@ static struct ns16550 {
 /* Frequency of external clock source. This definition assumes PC platform. */
 #define UART_CLOCK_HZ   1843200
 
+/* Resume retry settings */
+#define RESUME_DELAY    MILLISECS(10)
+#define RESUME_RETRIES  100
+
 static char ns_read_reg(struct ns16550 *uart, int reg)
 {
     if ( uart->remapped_io_base == NULL )
@@ -351,7 +356,7 @@ static void ns16550_suspend(struct serial_port *port)
                                   uart->ps_bdf[2], PCI_COMMAND);
 }
 
-static void ns16550_resume(struct serial_port *port)
+static void __ns16550_resume(struct serial_port *port)
 {
     struct ns16550 *uart = port->uart;
 
@@ -367,6 +372,58 @@ static void ns16550_resume(struct serial_port *port)
     ns16550_setup_postirq(port->uart);
 }
 
+static int ns16550_ioport_invalid(struct ns16550 *uart)
+{
+    return ((((unsigned char)ns_read_reg(uart, LSR)) == 0xff) &&
+            (((unsigned char)ns_read_reg(uart, MCR)) == 0xff) &&
+            (((unsigned char)ns_read_reg(uart, IER)) == 0xff) &&
+            (((unsigned char)ns_read_reg(uart, IIR)) == 0xff) &&
+            (((unsigned char)ns_read_reg(uart, LCR)) == 0xff));
+}
+
+static int delayed_resume_tries;
+static void ns16550_delayed_resume(void *data)
+{
+    struct serial_port *port = data;
+    struct ns16550 *uart = port->uart;
+
+    if ( ns16550_ioport_invalid(port->uart) && delayed_resume_tries-- )
+    {
+        set_timer(&uart->resume_timer, NOW() + RESUME_DELAY);
+        return;
+    }
+
+    __ns16550_resume(port);
+}
+
+static void ns16550_resume(struct serial_port *port)
+{
+    struct ns16550 *uart = port->uart;
+
+    /*
+     * Check for ioport access, before fully resuming operation.
+     * On some systems, there is a SuperIO card that provides
+     * this legacy ioport on the LPC bus.
+     *
+     * We need to wait for dom0's ACPI processing to run the proper
+     * AML to re-initialize the chip, before we can use the card again.
+     *
+     * This may cause a small amount of garbage to be written
+     * to the serial log while we wait patiently for that AML to
+     * be executed. However, this is preferable to spinning in an
+     * infinite loop, as seen on a Lenovo T430, when serial was enabled.
+     */
+    if ( ns16550_ioport_invalid(uart) )
+    {
+        delayed_resume_tries = RESUME_RETRIES;
+        init_timer(&uart->resume_timer, ns16550_delayed_resume, port, 0);
+        set_timer(&uart->resume_timer, NOW() + RESUME_DELAY);
+        return;
+    }
+
+    __ns16550_resume(port);
+}
+
 #ifdef CONFIG_X86
 static void __init ns16550_endboot(struct serial_port *port)
 {
-- 
1.7.9.5


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