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

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Date:	Wed, 23 Apr 2014 14:21:03 +0000
From:	"Karicheri, Muralidharan" <m-karicheri2@...com>
To:	"Karicheri, Muralidharan" <m-karicheri2@...com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>
CC:	"Balbi, Felipe" <balbi@...com>, Rob Herring <robh+dt@...nel.org>,
	"Mark Rutland" <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>
Subject: RE: [PATCH] serial: uart: add hw flow control support configuration

Alan,

Sorry, I had to cut-n-paste your comment as I lost your email from my inbox.

>-----Original Message-----
>From: Karicheri, Muralidharan
>Sent: Friday, April 11, 2014 3:18 PM
>To: devicetree@...r.kernel.org; linux-doc@...r.kernel.org; linux-kernel@...r.kernel.org;
>linux-serial@...r.kernel.org
>Cc: Balbi, Felipe; Karicheri, Muralidharan; Rob Herring; Mark Rutland; Ian Campbell;
>Kumar Gala; Randy Dunlap; Greg Kroah-Hartman; Jiri Slaby
>Subject: [PATCH] serial: uart: add hw flow control support configuration
>
>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.

[Alan] That seems a sensible thing to add.

>
>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>
>
>Signed-off-by: Murali Karicheri <m-karicheri2@...com>
>---
> .../devicetree/bindings/serial/of-serial.txt       |    2 ++
> 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, 19 insertions(+), 5 deletions(-)
>
>diff --git a/Documentation/devicetree/bindings/serial/of-serial.txt
>b/Documentation/devicetree/bindings/serial/of-serial.txt
>index 1928a3e..123de01 100644
>--- a/Documentation/devicetree/bindings/serial/of-serial.txt
>+++ b/Documentation/devicetree/bindings/serial/of-serial.txt
>@@ -37,6 +37,8 @@ Optional properties:
> - auto-flow-control: one way to enable automatic flow control support. The
>   driver is allowed to detect support for the capability even without this
>   property.
>+- hw-flow-control: this enables pure hw flow control and no software
>+  intervention needed.
>
> Example:
>
>diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
>index 81f909c..6b1b868 100644
>--- a/drivers/tty/serial/8250/8250_core.c
>+++ b/drivers/tty/serial/8250/8250_core.c
>@@ -2338,9 +2338,11 @@ serial8250_do_set_termios(struct uart_port *port, struct
>ktermios *termios,
> 	 * the trigger, or the MCR RTS bit is cleared.  In the case where
> 	 * the remote UART is not using CTS auto flow control, we must
> 	 * have sufficient FIFO entries for the latency of the remote
>-	 * UART to respond.  IOW, at least 32 bytes of FIFO.
>+	 * UART to respond.  IOW, at least 32 bytes of FIFO. Also enable
>+	 * AFE if hw flow control is supported
> 	 */
>-	if (up->capabilities & UART_CAP_AFE && port->fifosize >= 32) {
>+	if ((up->capabilities & UART_CAP_AFE && (port->fifosize >= 32)) ||
>+	    port->flags & UPF_HARD_FLOW) {

[Alan] Surely you want brackets on the port->flags & UPF_HARD_FLOW ??

The C precedence table shows following order:=
-> & ||

So flags will be accessed first and then AND-ed with  UPF_HARD_FLOW and
result will be OR-ed with left side expression value. So no need for bracket IMO.

Murali
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