[RFC PATCH v3] tty: pl011: Avoid spuriously stuck-off interrupts

[Dave Martin]

Commit 9b96fbacda34 ("serial: PL011: clear pending interrupts")
clears the RX and receive timeout interrupts on pl011 startup, to
avoid a screaming-interrupt scenario that can occur when the
firmware or bootloader leaves these interrupts asserted.

This has been noted as an issue when running Linux on qemu [1].

Unfortunately, the above fix seems to lead to potential
misbehaviour if the RX FIFO interrupt is asserted _non_ spuriously
on driver startup, if the RX FIFO is also already full to the
trigger level.

Clearing the RX FIFO interrupt does not change the FIFO fill level.
In this scenario, because the interrupt is now clear and because
the FIFO is already full to the trigger level, no new assertion of
the RX FIFO interrupt can occur unless the FIFO is drained back
below the trigger level.  This never occurs because the pl011
driver is waiting for an RX FIFO interrupt to tell it that there is
something to read, and does not read the FIFO at all until that
interrupt occurs.

Thus, simply clearing "spurious" interrupts on startup may be
misguided, since there is no way to be sure that the interrupts are
truly spurious, and things can go wrong if they are not.

This patch instead clears the interrupt condition by draining the
RX FIFO during UART startup, after clearing any potentially
spurious interrupt.  This should ensure that an interrupt will
definitely be asserted if the RX FIFO subsequently becomes
sufficiently full.

This patch does not attempt to address the case where the RX FIFO
fills faster than it can be drained: that is a pathological
condition that is beyond the scope of the driver to work around.
Users cannot expect this to work unless they enable hardware flow
control.

[1] [Qemu-devel] [Qemu-arm] [PATCH] pl011: do not put into fifo
before enabled the interruption
https://lists.gnu.org/archive/html/qemu-devel/2018-01/msg06446.html

Reported-by: Wei Xu <xuwei5 at hisilicon.com>
Cc: Russell King <linux at armlinux.org.uk>
Cc: Linus Walleij <linus.walleij at linaro.org>
Cc: Peter Maydell <peter.maydell at linaro.org>
Fixes: 9b96fbacda34 ("serial: PL011: clear pending interrupts")
Signed-off-by: Dave Martin <Dave.Martin at arm.com>

---

Changes since RFC v2:

 * Drain the RX FIFO explicitly on startup.

   This reduces the complexity of the changes, but should still
   fix the underlying problem.

   Characters present in the RX FIFO before the kernel boots will
   now be lost.  However it was always poor practice to rely on
   such characters being processed by the kernel, since UART
   initialisation and RX FIFO overruns can always lead to data
   loss or corruption in any case.
---
 drivers/tty/serial/amba-pl011.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 4b40a5b..73e6f44 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -1676,6 +1676,15 @@ static int pl011_hwinit(struct uart_port *port)
 		    uap, REG_ICR);
 
 	/*
+	 * RXIS is asserted only when the RX FIFO transitions from below
+	 * to above the trigger threshold.  If the RX FIFO is already
+	 * full to the threshold this can't happen and RXIS will now be
+	 * stuck off.  Drain the FIFO explicitly to fix this:
+	 */
+	while (!(pl011_read(uap, REG_FR) & UART01x_FR_RXFE))
+		pl011_read(uap, REG_DR);
+
+	/*
 	 * Save interrupts enable mask, and enable RX interrupts in case if
 	 * the interrupt is used for NMI entry.
 	 */
-- 
2.1.4