[PATCH 1/2] spi: Add a DT binding for high-impedance turn-around

[PATCH 1/2] spi: Add a DT binding for high-impedance turn-around

[Linus Walleij]

Some 3WIRE SPI devices require the host to insert a
"high-impedance turn-around" essentially a clock pulse after
switching the one line from output to input.

This is needed to support the TPO TPG110 panel to use
the 3WIRE SPI bindings.

Cc: devicetree@vger.kernel.org
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
ChangeLog v1->v2:
- Change some spelling, instert dash in high-impedance
  use turn-around instead of turnaround.
---
 Documentation/devicetree/bindings/spi/spi-bus.txt | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Documentation/devicetree/bindings/spi/spi-bus.txt b/Documentation/devicetree/bindings/spi/spi-bus.txt
index 1f6e86f787ef..d0b2d88a20ee 100644
--- a/Documentation/devicetree/bindings/spi/spi-bus.txt
+++ b/Documentation/devicetree/bindings/spi/spi-bus.txt
@@ -70,6 +70,10 @@ All slave nodes can contain the following optional properties:
 - spi-cs-high     - Empty property indicating device requires chip select
 		    active high.
 - spi-3wire       - Empty property indicating device requires 3-wire mode.
+- spi-3wire-high-impedance-turn-around - Empty property indicating that a
+		    3wire host need to insert a high-impedance turn-around
+		    clock cycle after turning the one output line into an
+		    input line.
 - spi-lsb-first   - Empty property indicating device requires LSB first mode.
 - spi-tx-bus-width - The bus width (number of data wires) that is used for MOSI.
 		    Defaults to 1 if not present.
-- 
2.17.1

Re: [PATCH 1/2] spi: Add a DT binding for high-impedance turn-around

[Mark Brown]

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On Tue, Sep 25, 2018 at 04:32:19PM +0200, Linus Walleij wrote:
> Some 3WIRE SPI devices require the host to insert a
> "high-impedance turn-around" essentially a clock pulse after
> switching the one line from output to input.

> +- spi-3wire-high-impedance-turn-around - Empty property indicating that a
> +		    3wire host need to insert a high-impedance turn-around
> +		    clock cycle after turning the one output line into an
> +		    input line.

Should this not be a flag set through the API rather than a DT property?
It doesn't look like something that's per system configurable but rather
something that the device is always going to need to do so every single
usage of the device is going to need this property.

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Re: [PATCH 1/2] spi: Add a DT binding for high-impedance turn-around

[Linus Walleij]

On Fri, Sep 28, 2018 at 3:18 PM Mark Brown <broonie@kernel.org> wrote:
> On Tue, Sep 25, 2018 at 04:32:19PM +0200, Linus Walleij wrote:
> > Some 3WIRE SPI devices require the host to insert a
> > "high-impedance turn-around" essentially a clock pulse after
> > switching the one line from output to input.
>
> > +- spi-3wire-high-impedance-turn-around - Empty property indicating that a
> > +                 3wire host need to insert a high-impedance turn-around
> > +                 clock cycle after turning the one output line into an
> > +                 input line.
>
> Should this not be a flag set through the API rather than a DT property?
> It doesn't look like something that's per system configurable but rather
> something that the device is always going to need to do so every single
> usage of the device is going to need this property.

Good point. Let me take it for a spin using some flag inside Linux
instead and see if it looks better.

Yours,
Linus Walleij