[PATCH] tty/serial: atmel_serial: BUG: stop DMA from transmitting in stop_tx

[Richard Genoud]

2017-03-15 12:37 GMT+01:00 Nicolas Ferre <nicolas.ferre at atmel.com>:
> Le 13/12/2016 à 17:27, Richard Genoud a écrit :
>> 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: <stable at vger.kernel.org> (beware, this won't apply before 4.3)
>> Signed-off-by: Richard Genoud <richard.genoud at gmail.com>
>> ---
>>  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...
aïe !
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.