linux-kernel - Re: [STLinux Kernel] [PATCH 8/8] ARM: dts: STiH407-family: Enable HW flow-control

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Message-ID: <20170127113252.dhuflg2qrl3kobda@dell>
Date:   Fri, 27 Jan 2017 11:32:52 +0000
From:   Lee Jones <lee.jones@...aro.org>
To:     Peter Griffin <peter.griffin@...aro.org>
Cc:     gregkh@...uxfoundation.org, jslaby@...e.com,
        linux-serial@...r.kernel.org, robh+dt@...nel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kernel@...inux.com
Subject: Re: [STLinux Kernel] [PATCH 8/8] ARM: dts: STiH407-family: Enable HW
 flow-control

On Wed, 25 Jan 2017, Peter Griffin wrote:

> Hi Lee,
> 
> On Tue, 24 Jan 2017, Lee Jones wrote:
> 
> > Hardware flow-control capability must be specified at a platform
> > level in order to inform the ASC driver that the platform is capable
> > (i.e. are the lines wired up, etc).  STiH4{07,10} devices are indeed
> > capable, so let's provide the property.
> > 
> > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> > ---
> >  arch/arm/boot/dts/stih407-family.dtsi | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm/boot/dts/stih407-family.dtsi b/arch/arm/boot/dts/stih407-family.dtsi
> > index 9789978..7ada8ea 100644
> > --- a/arch/arm/boot/dts/stih407-family.dtsi
> > +++ b/arch/arm/boot/dts/stih407-family.dtsi
> > @@ -226,7 +226,7 @@
> >  			pinctrl-0 = <&pinctrl_serial0_flowctrl>;
> >  			pinctrl-1 = <&pinctrl_serial0>;
> >  			clocks = <&clk_s_c0_flexgen CLK_EXT2F_A9>;
> > -
> > +			st,hw-flow-control;
> 
> There is a generic serial binding for this already. As this ST property
> hasn't been used upstream, it seems like it would be worth dropping it
> and switching to the generic uart-has-rtscts one.
> 
> See Documentation/devicetree/bindings/serial/serial.txt
> 
>   - uart-has-rtscts: The presence of this property indicates that the
>     UART has dedicated lines for RTS/CTS hardware flow control, and that
>     they are available for use (wired and enabled by pinmux configuration).
>     This depends on both the UART hardware and the board wiring.
>     Note that this property is mutually-exclusive with "cts-gpios" and
>     "rts-gpios" above.

I've done some digging and I can't see anywhere where
st,hw-flow-control is being used, even in the BSP kernel(s).  It's
also not documented in dt-bindings.  With that in mind, I think it's
probably okay to use the generic binding.

Although, the "Note" at the bottom of the uart-has-rtscts is not
correct in our case, since we handle the case dynamically.

> Also you should put this in the board dtsi, as it is board dependent property.
> By putting it here you are enabling hw-flow-control for all stih407-family
> based boards.

Fair shout.
  
On the B2120 UART0 is hooked up to the Smart Card Reader.
  
Will fix.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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