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

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Date:	Thu, 7 Aug 2014 13:34:04 -0400
From:	Murali Karicheri <m-karicheri2@...com>
To:	Peter Hurley <peter@...leysoftware.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<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>,
	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 01:25 PM, Peter Hurley wrote:
> On 08/07/2014 01:12 PM, Greg Kroah-Hartman wrote:
>> On Thu, Aug 07, 2014 at 12:16:59PM -0400, Murali Karicheri wrote:
>>> 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.
>>
>> If someone points out a problem in a patch of yours that is accepted
>> upstream, it is nice to provide a fix, otherwise I will just revert it
>> upstream, as it looks to be incorrect.
>>
>> So, should I just revert it?
>
> Greg,
>
> As I look this patch over further, this should just be reverted.

Sorry, I would suggest to fix it rather revert it.

>
> 1. The patch enables UPF_HARD_FLOW, but provides no throttle() and unthrottle()
> methods for 8250, which is guaranteed to blow-up when either uart_throttle() or
> uart_unthrottle() is called.
>
> 2. The patch adds capabilities which already exist, namely UART_CAP_AFE.
AFAIK, UART_CAP_AFE is a software assisted hw flow control and it was 
described in my commit log as well where as this patch add support for 
pure h/w controlled flow  control and no software intervention is 
needed. Do you think uart_throttle() or  uart_unthrottle() is applicable
in this case?

Murali
>
> Regards,
> Peter Hurley
>

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