8250: improve workaround for UARTs that don't re-assert THRE correctly

Recent changes to tighten the check for UARTs that don't correctly
re-assert THRE (01c194d9278efc15d4785ff205643e9c0bdcef53: "serial 8250:
tighten test for using backup timer") caused problems when such a UART was
opened for the second time - the bug could only successfully be detected
at first initialization.  For users of this version of this particular
UART IP it is fatal.

This patch stores the information about the bug in the bugs field of the
port structure when the port is first started up so subsequent opens can
check this bit even if the test for the bug fails.

David Brownell: "My own exposure to this is that the UART on DaVinci
hardware, which TI allegedly derived from its original 16550 logic, has
periodically gone from working to unusable with the mainline 8250.c ...
and back and forth a bunch.  Currently it's "unusable", a regression from
some previous versions.  With this patch from Will, it's usable."

Signed-off-by: Will Newton <will.newton@gmail.com>
Acked-by: Alex Williamson <alex.williamson@hp.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: David Brownell <david-b@pacbell.net>
Cc: <stable@kernel.org>		[2.6.26.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
index 342e12fb1c2525f74bb15d0e21ca4e473962ecd1..9ccc563d87303604c8cd63f355040177c30faf81 100644 (file)
--- a/drivers/serial/8250.c
+++ b/drivers/serial/8250.c
@@ -1908,14 +1908,22 @@ static int serial8250_startup(struct uart_port *port)
                 * kick the UART on a regular basis.
                 */
                if (!(iir1 & UART_IIR_NO_INT) && (iir & UART_IIR_NO_INT)) {
+                       up->bugs |= UART_BUG_THRE;
                        pr_debug("ttyS%d - using backup timer\n", port->line);
-                       up->timer.function = serial8250_backup_timeout;
-                       up->timer.data = (unsigned long)up;
-                       mod_timer(&up->timer, jiffies +
-                               poll_timeout(up->port.timeout) + HZ / 5);
                }
        }
 
+       /*
+        * The above check will only give an accurate result the first time
+        * the port is opened so this value needs to be preserved.
+        */
+       if (up->bugs & UART_BUG_THRE) {
+               up->timer.function = serial8250_backup_timeout;
+               up->timer.data = (unsigned long)up;
+               mod_timer(&up->timer, jiffies +
+                         poll_timeout(up->port.timeout) + HZ / 5);
+       }
+
        /*
         * If the "interrupt" for this port doesn't correspond with any
         * hardware interrupt, we use a timer-based system.  The original

diff --git a/drivers/serial/8250.h b/drivers/serial/8250.h
index 78c00162b04e7630bd5a08f3df952dd441e8e503..520260326f3dfeb64b67643e1a27938e733d71fc 100644 (file)
--- a/drivers/serial/8250.h
+++ b/drivers/serial/8250.h
@@ -47,6 +47,7 @@ struct serial8250_config {
 #define UART_BUG_QUOT  (1 << 0)        /* UART has buggy quot LSB */
 #define UART_BUG_TXEN  (1 << 1)        /* UART has buggy TX IIR status */
 #define UART_BUG_NOMSR (1 << 2)        /* UART has buggy MSR status bits (Au1x00) */
+#define UART_BUG_THRE  (1 << 3)        /* UART has buggy THRE reassertion */
 
 #define PROBE_RSA      (1 << 0)
 #define PROBE_ANY      (~0)