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SubjectRe: [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@ti.com>
>>>>>
>>>>> CC: Rob Herring<robh+dt@kernel.org>
>>>>> CC: Pawel Moll<pawel.moll@arm.com>
>>>>> CC: Mark Rutland<mark.rutland@arm.com>
>>>>> CC: Ian Campbell<ijc+devicetree@hellion.org.uk>
>>>>> CC: Kumar Gala<galak@codeaurora.org>
>>>>> CC: Randy Dunlap<rdunlap@infradead.org>
>>>>> CC: Greg Kroah-Hartman<gregkh@linuxfoundation.org>
>>>>> CC: Jiri Slaby<jslaby@suse.cz>
>>>>> CC: Santosh Shilimkar<santosh.shilimkar@ti.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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