[PATCH 09/16] tty: serial: 8250_dma: Add a TX trigger workaround for AM33xx

Peter Hurley peter at hurleysoftware.com
Thu Sep 11 07:35:43 PDT 2014

On 09/11/2014 08:50 AM, Sebastian Andrzej Siewior wrote:
> On 09/11/2014 02:32 PM, Peter Hurley wrote:
>> On 09/11/2014 07:42 AM, Sebastian Andrzej Siewior wrote:
>>> I also need a watchdog timer for TX since it seems that on omap3 the
>>> DMA engine suddenly forgets to continue with DMA…
>> One difference I noticed between the omap driver and the 8250 driver is
>> the way modem status interrupts are handled.
>> The omap driver only checks modem status for the UART_IIR_MSI interrupt type.
>> The 8250 driver checks modem status at every interrupt (other than NO_INT).
>> I think the UART_MSR_DCTS bit always reflects that the CTS input has changed
>> between reads of the MSR _even if auto CTS is on_. So perhaps the hardware
>> is being stopped by uart_handle_cts_change() when auto CTS is on?
> I doubt that. What I see from a timer debug is that the TX-FIFO level
> is at 0, the DMA transfer for say 1024 bytes start.
> The FIFO is filled to 64bytes and refilled so it doesn't drop below 50.
> At the time of the stall I see that the DMA engine has outstanding
> bytes which it should transfer and the TX FIFO is empty. If hardware
> flow control stops the transfer, I would expect that the DMA engine
> still fills the TX-FIFO until 64 and then waits. But it doesn't.
> Writing bytes into the FIFO leads to bytes beeing sent (and I see them
> on the other side) but the DMA transfer is still on hold. Canceling the
> DMA transfer and re-programming the remaining bytes transfers the
> remaining bytes.
> The odd thing is that I only triggered it with "less file". It doesn't
> happen on regular console interaction or "cat large-file". And it only
> triggers on beagle board xm (omap34xx) and I wasn't able to reproduce
> it on am335x or dra7. The latter shares the same DMA engine as beagle
> board xm.
> I remember also that I disabled the HW/SW float control just to make
> sure it is not it.


I do need to find out if omap hardware sets UART_MSR_DCTS when auto CTS
is on. Would you mind running the debug patch below with HW flow control on?

Peter Hurley

--- >% ---
Subject: [DEBUG] serial: does OMAP set UART_LSR_DCTS while autoCTS is on?

** debug patch only **

Signed-off-by: Peter Hurley <peter at hurleysoftware.com>
 drivers/tty/serial/serial_core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 5a78f69..1579a20 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -2783,6 +2783,9 @@ void uart_handle_cts_change(struct uart_port *uport, unsigned int status)
 	if (tty_port_cts_enabled(port)) {
+		WARN_ON_ONCE(uport->flags & UPF_HARD_FLOW);
 		if (tty->hw_stopped) {
 			if (status) {
 				tty->hw_stopped = 0;

More information about the linux-arm-kernel mailing list