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Date:	Thu, 07 Aug 2014 14:33:41 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Murali Karicheri <m-karicheri2@...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 01:34 PM, Murali Karicheri wrote:
> 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?

UART_CAP_AFE is used to indicate 16750-compatible hw flow control, which is
auto-CTS and auto-RTS flow control as described in the TI datasheet at
http://www.ti.com/lit/ds/symlink/tl16c750.pdf

uart_throttle() and uart_unthrottle() are used indirectly by line disciplines
for high-level rx flow control, such as when a read buffer fills up because
there is no userspace reader. The 8250 core doesn't define a throttle/unthrottle
method because writing MCR to drop RTS is sufficient to disable auto-RTS.

Regards,
Peter Hurley

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