serial: fix lost character after sysrq

After creating an automated regression test to test the sysrq responses while running a linux image in qemu, I found that the simulated uart was eating the character right after the sysrq about 75% of the time. The problem is that the qemu sets the LSR_DR (data ready) bit on a serial break. The automated tests can send a break and the sysrq character quickly enough that the qemu serial fifo has a real character available. When there is valid character in the fifo, it gets consumed by the serial driver in the guest OS. The real hardware also appears to set the LSR_DR but always appears to have a null byte in this condition. This patch changes the qemu behavior to match the tested characteristics of a real 16550 chip. Signed-off-by: Jason Wessel <jason.wessel@windriver.com>

serial: fix lost character after sysrq
After creating an automated regression test to test the sysrq responses while running a linux image in qemu, I found that the simulated uart was eating the character right after the sysrq about 75% of the time. The problem is that the qemu sets the LSR_DR (data ready) bit on a serial break. The automated tests can send a break and the sysrq character quickly enough that the qemu serial fifo has a real character available. When there is valid character in the fifo, it gets consumed by the serial driver in the guest OS. The real hardware also appears to set the LSR_DR but always appears to have a null byte in this condition. This patch changes the qemu behavior to match the tested characteristics of a real 16550 chip. Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Jason Wessel · Anthony Liguori
1 parent 7e57f049
Showing 1 changed file with 2 additions and 0 deletions
hw/serial.c
@@ -586,6 +586,8 @@ static int serial_can_receive(SerialState *s)
 static void serial_receive_break(SerialState *s)
 {
     s->rbr = 0;
+    /* When the LSR_DR is set a null byte is pushed into the fifo */
+    fifo_put(s, RECV_FIFO, '\0');
     s->lsr |= UART_LSR_BI | UART_LSR_DR;
     serial_update_irq(s);
 }