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

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Date:	Thu, 07 Aug 2014 13:25:18 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Murali Karicheri <m-karicheri2@...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>,
	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 01:12 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.

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.

Regards,
Peter Hurley

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