This is a note to let you know that I've just added the patch titled

tty: pl011: Avoid spuriously stuck-off interrupts

to my tty git tree which can be found at git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git in the tty-testing branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will be merged to the tty-next branch sometime soon, after it passes testing, and the merge window is open.

If you have any questions about this process, please let me know.

From 4a7e625ce50412a7711efa0f2ef0b96ce3826759 Mon Sep 17 00:00:00 2001

From: Dave Martin Dave.Martin@arm.com Date: Thu, 10 May 2018 18:08:23 +0100 Subject: tty: pl011: Avoid spuriously stuck-off interrupts

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.

The drain is done at the point of enabling interrupts only. This means that it will occur any time the UART is newly opened through the tty layer. It will not apply to polled-mode use of the UART by kgdboc: since that scenario cannot use interrupts by design, this should not matter. kgdboc will interact badly with "normal" use of the UART in any case: this patch makes no attempt to paper over such issues.

This patch does not attempt to address the case where the RX FIFO fills faster than it can be drained: that is a pathological hardware design problem that is beyond the scope of the driver to work around. As a failsafe, the number of poll iterations for draining the FIFO is limited to twice the FIFO size. This will ensure that the kernel at least boots even if it is impossible to drain the FIFO for some reason.

[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@hisilicon.com Cc: Russell King linux@armlinux.org.uk Cc: Linus Walleij linus.walleij@linaro.org Cc: Peter Maydell peter.maydell@linaro.org Fixes: 9b96fbacda34 ("serial: PL011: clear pending interrupts") Signed-off-by: Dave Martin Dave.Martin@arm.com Cc: stable stable@vger.kernel.org Tested-by: Wei Xu xuwei5@hisilicon.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/tty/serial/amba-pl011.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c index 4b40a5b449ee..ebd33c0232e6 100644 --- a/drivers/tty/serial/amba-pl011.c +++ b/drivers/tty/serial/amba-pl011.c @@ -1727,10 +1727,26 @@ static int pl011_allocate_irq(struct uart_amba_port *uap) */ static void pl011_enable_interrupts(struct uart_amba_port *uap) { + unsigned int i; + spin_lock_irq(&uap->port.lock);

/* Clear out any spuriously appearing RX interrupts */ pl011_write(UART011_RTIS | UART011_RXIS, 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 RX FIFO explicitly to fix this: + */ + for (i = 0; i < uap->fifosize * 2; ++i) { + if (pl011_read(uap, REG_FR) & UART01x_FR_RXFE) + break; + + pl011_read(uap, REG_DR); + } + uap->im = UART011_RTIM; if (!pl011_dma_rx_running(uap)) uap->im |= UART011_RXIM;