linux-kernel - Re: [PATCH v2] serial: uart: add hw flow control support configuration

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Date:	Thu, 7 Aug 2014 12:16:59 -0400
From:	Murali Karicheri <m-karicheri2@...com>
To:	Peter Hurley <peter@...leysoftware.com>
CC:	<devicetree@...r.kernel.org>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-serial@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Santosh Shilimkar <santosh.shilimkar@...com>
Subject: Re: [PATCH v2] serial: uart: add hw flow control support configuration

On 08/07/2014 11:29 AM, Peter Hurley wrote:
> On 05/01/2014 03:04 PM, Murali Karicheri wrote:
>> 8250 uart driver currently supports only software assisted hw flow
>> control. The software assisted hw flow control maintains a hw_stopped
>> flag in the tty structure to stop and start transmission and use modem
>> status interrupt for the event to drive the handshake signals. This is
>> not needed if hw has flow control capabilities. This patch adds a
>> DT attribute for enabling hw flow control for a uart port. Also skip
>> stop and start if this flag is present in flag field of the port
>> structure.
>>
>> Signed-off-by: Murali Karicheri<m-karicheri2@...com>
>>
>> CC: Rob Herring<robh+dt@...nel.org>
>> CC: Pawel Moll<pawel.moll@....com>
>> CC: Mark Rutland<mark.rutland@....com>
>> CC: Ian Campbell<ijc+devicetree@...lion.org.uk>
>> CC: Kumar Gala<galak@...eaurora.org>
>> CC: Randy Dunlap<rdunlap@...radead.org>
>> CC: Greg Kroah-Hartman<gregkh@...uxfoundation.org>
>> CC: Jiri Slaby<jslaby@...e.cz>
>> CC: Santosh Shilimkar<santosh.shilimkar@...com>
>> ---
>>   .../devicetree/bindings/serial/of-serial.txt       |    1 +
>>   drivers/tty/serial/8250/8250_core.c                |    6 ++++--
>>   drivers/tty/serial/of_serial.c                     |    4 ++++
>>   drivers/tty/serial/serial_core.c                   |   12 +++++++++---
>>   4 files changed, 18 insertions(+), 5 deletions(-)
>>
> [...]
>
>> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
>> index b68550d..851707a 100644
>> --- a/drivers/tty/serial/serial_core.c
>> +++ b/drivers/tty/serial/serial_core.c
>> @@ -174,8 +174,12 @@ static int uart_port_startup(struct tty_struct *tty, struct uart_state *state,
>>   			if (tty->termios.c_cflag&  CBAUD)
>>   				uart_set_mctrl(uport, TIOCM_RTS | TIOCM_DTR);
>>   		}
>> -
>> -		if (tty_port_cts_enabled(port)) {
>> +		/*
>> +		 * if hw support flow control without software intervention,
>> +		 * then skip the below check
>> +		 */
>> +		if (tty_port_cts_enabled(port)&&
>> +		    !(uport->flags&  UPF_HARD_FLOW)) {
>>   			spin_lock_irq(&uport->lock);
>>   			if (!(uport->ops->get_mctrl(uport)&  TIOCM_CTS))
>>   				tty->hw_stopped = 1;
>> @@ -2772,7 +2776,9 @@ void uart_handle_cts_change(struct uart_port *uport, unsigned int status)
>>
>>   	uport->icount.cts++;
>>
>> -	if (tty_port_cts_enabled(port)) {
>> +	/* skip below code if the hw flow control is supported */
>> +	if (tty_port_cts_enabled(port)&&
>> +	    !(uport->flags&  UPF_HARD_FLOW)) {
> Why is a modem status interrupt being generated for DCTS
> if autoflow control is enabled?
>
> This should be:
>
> 	WARN_ON_ONCE(uport->flags&  UPF_HARD_FLOW);
>
> to indicate a mis-configured driver/device.
This patch is already merged to the upstream branch and if you see any 
issue, please
post a patch for review.

Murali
>
>>   		if (tty->hw_stopped) {
>>   			if (status) {
>>   				tty->hw_stopped = 0;
>>

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