From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Hurley Subject: Re: [PATCH v2] serial: uart: add hw flow control support configuration Date: Thu, 07 Aug 2014 14:33:41 -0400 Message-ID: <53E3C685.5010600@hurleysoftware.com> References: <1398971093-17164-1-git-send-email-m-karicheri2@ti.com> <53E39B44.60406@hurleysoftware.com> <53E3A67B.6070709@ti.com> <20140807171254.GA13097@kroah.com> <53E3B67E.80705@hurleysoftware.com> <53E3B88C.2000209@ti.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <53E3B88C.2000209@ti.com> Sender: linux-doc-owner@vger.kernel.org To: Murali Karicheri Cc: Greg Kroah-Hartman , devicetree@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, Rob Herring , Pawel Moll , Mark Rutland , Ian Campbell , Kumar Gala , Randy Dunlap , Jiri Slaby , Santosh Shilimkar List-Id: devicetree@vger.kernel.org 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 >>>>>> >>>>>> CC: Rob Herring >>>>>> CC: Pawel Moll >>>>>> CC: Mark Rutland >>>>>> CC: Ian Campbell >>>>>> CC: Kumar Gala >>>>>> CC: Randy Dunlap >>>>>> CC: Greg Kroah-Hartman >>>>>> CC: Jiri Slaby >>>>>> CC: Santosh Shilimkar >>>>>> --- >>>>>> .../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