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

[Karicheri, Muralidharan]

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@xxxxxxxxxxxxxxx; linux-doc@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
>linux-serial@xxxxxxxxxxxxxxx
>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@xxxxxxxxxx>
>CC: Pawel Moll <pawel.moll@xxxxxxx>
>CC: Mark Rutland <mark.rutland@xxxxxxx>
>CC: Ian Campbell <ijc+devicetree@xxxxxxxxxxxxxx>
>CC: Kumar Gala <galak@xxxxxxxxxxxxxx>
>CC: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
>CC: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>CC: Jiri Slaby <jslaby@xxxxxxx>
>
>Signed-off-by: Murali Karicheri <m-karicheri2@xxxxxx>
>---
> .../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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