2017-03-15 12:37 GMT+01:00 Nicolas Ferre [off-list ref]:

Le 13/12/2016 ? 17:27, Richard Genoud a ?crit :

quoted

If we don't disable the transmitter in atmel_stop_tx, the DMA buffer
continues to send data until it is emptied.
This cause problems with the flow control (CTS is asserted and data are
still sent).

So, disabling the transmitter in atmel_stop_tx is a sane thing to do.

Tested on at91sam9g35-cm(DMA)
Tested for regressions on sama5d2-xplained(Fifo) and at91sam9g20ek(PDC)

Cc: <redacted> (beware, this won't apply before 4.3)
Signed-off-by: Richard Genoud <redacted>
---
 drivers/tty/serial/atmel_serial.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

NB: this is not for the 4.10 merge window, I'm just sending it now to
have some comments if someone is againts it.

diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
index 168b10cad47b..f9d42de5ab2d 100644
--- a/drivers/tty/serial/atmel_serial.c
+++ b/drivers/tty/serial/atmel_serial.c

@@ -481,6 +481,14 @@ static void atmel_stop_tx(struct uart_port *port)
              /* disable PDC transmit */
              atmel_uart_writel(port, ATMEL_PDC_PTCR, ATMEL_PDC_TXTDIS);
      }
+
+     /*
+      * Disable the transmitter.
+      * This is mandatory when DMA is used, otherwise the DMA buffer
+      * is fully transmitted.
+      */
+     atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_TXDIS);
+
      /* Disable interrupts */
      atmel_uart_writel(port, ATMEL_US_IDR, atmel_port->tx_done_mask);

@@ -513,6 +521,9 @@ static void atmel_start_tx(struct uart_port *port)

      /* Enable interrupts */
      atmel_uart_writel(port, ATMEL_US_IER, atmel_port->tx_done_mask);
+
+     /* re-enable the transmitter */
+     atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_TXEN);
 }

 /*

Hi Richard,

Hi !

I've just discovered that I have some weird behavior with this patch. On
current Linus' tree, with sama5d2 + DMA, I see some garbage characters
coming out of the console when I try to stop my system (reboot/halt) [1].

Yes, I've also seen that on my board, it was on my todo list.
(Although I didn't know this patch was the culprit !)

Moreover, and I do understand that it's not the problem right here, when
applied on our linux-4.4-at91 branch (our vendor tree actually), it
hangs the boot process as it seems that a burst of open/close of the
serial port happens while starting the rootfs. It's definitively my own
problem, but it can bring light to what we are seeing on Mainline...

a?e !
This is clearly something we'll have to understand !

I think that we may need to flush the DMA channel in this
atmel_stop_tx() function.

yes, I'll look into that.

Best regards,

[1] If you want to test, you need to apply this patch for eMMC BTW:
https://patchwork.kernel.org/patch/9617489/

--
Nicolas Ferre

Thanks !

Richard.