[Qemu-stable] [PATCH 3/3] serial: do not trigger THR interrupt after writing to IER

[Paolo Bonzini]

This is responsible for failure of migration from 2.2 to 2.1, because
thr_ipending is always one in practice.  Calling serial_update_irq is
the right thing to do indeed, because writing to IER could cause an
interrupt to appear.  However, there is no reason to set thr_ipending
again.

This was already reported in 2010.  See this quote from
https://lists.gnu.org/archive/html/qemu-devel/2010-03/msg01914.html:

> The commit in r1049 (serial interrupt fix (Hampa Hug)) prevents
> booting Digital Research DOSPlus. Following patch partially reverts
> that commit and makes DOSPlus booting in QEMU again.

Bochs does not check LSR_THRE in IER, and the log message in r1049 doesn't
explain why the change was made in the first place.

This does not change the migration format, so 2.2.0 -> 2.1 will remain
broken but we can fix 2.2.1 -> 2.1 without breaking 2.2.1 <-> 2.2.0.

Cc: address@hidden
Reported-by: Igor Mammedov <address@hidden>
Reported-by: Roy Tam <address@hidden>
Signed-off-by: Paolo Bonzini <address@hidden>
---
 hw/char/serial.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/hw/char/serial.c b/hw/char/serial.c
index f35fa42..2b7f4f5 100644
--- a/hw/char/serial.c
+++ b/hw/char/serial.c
@@ -348,10 +348,7 @@ static void serial_ioport_write(void *opaque, hwaddr addr, 
uint64_t val,
                      s->poll_msl = 0;
                 }
             }
-            if (s->lsr & UART_LSR_THRE) {
-                s->thr_ipending = 1;
-                serial_update_irq(s);
-            }
+            serial_update_irq(s);
         }
         break;
     case 2:
-- 
1.8.3.1