Re: [PATCH v4 5/9] spi: Documentation: add page on multi-lane support

[Marcelo Schmitt]

Hi David,

Thanks for adding a doc for the multi-lane stuff.
Two minor comments inline.

Reviewed-by: Marcelo Schmitt <marcelo.schmitt@xxxxxxxxxx>

On 12/19, David Lechner wrote:
> Add a new page to Documentation/spi/ describing how multi-lane SPI
> support works. This is uncommon functionality so it deserves its own
> documentation page.
> 
> Signed-off-by: David Lechner <dlechner@xxxxxxxxxxxx>
> ---
> v4 changes:
> * New patch in v4.
> ---
>  Documentation/spi/index.rst               |   1 +
>  Documentation/spi/multiple-data-lanes.rst | 217 ++++++++++++++++++++++++++++++
>  2 files changed, 218 insertions(+)
> 
> diff --git a/Documentation/spi/index.rst b/Documentation/spi/index.rst
> index 824ce42ed4f0..2c89b1ee39e2 100644
> --- a/Documentation/spi/index.rst
> +++ b/Documentation/spi/index.rst
> @@ -9,6 +9,7 @@ Serial Peripheral Interface (SPI)
>  
>     spi-summary
>     spidev
> +   multiple-data-lanes
>     butterfly
>     spi-lm70llp
>     spi-sc18is602
> diff --git a/Documentation/spi/multiple-data-lanes.rst b/Documentation/spi/multiple-data-lanes.rst
> new file mode 100644
> index 000000000000..b267f31f0bc8
> --- /dev/null
> +++ b/Documentation/spi/multiple-data-lanes.rst
> @@ -0,0 +1,217 @@
> +====================================
> +SPI devices with multiple data lanes
> +====================================
> +
> +Some specialized SPI controllers and peripherals support multiple data lanes
> +that allow reading more than one word at a time in parallel. This is different
> +from dual/quad/octal SPI where multiple bits of a single word are transferred
> +simultaneously.
> +
> +For example, controllers that support parallel flash memories have this feature
> +as do some simultaneous-sampling ADCs where each channel has its own data lane.
> +
> +---------------------
> +Describing the wiring
> +---------------------
> +
> +The ``spi-tx-bus-width`` and ``spi-rx-bus-width`` properties in the devicetree
> +are used to describe how many data lanes are connected between the controller
> +and how wide each lane is. The number of items in the array indicates how many
> +lanes there are, and the value of each item indicates how many bits wide that
> +lane is.
> +
> +For example, a dual-simultaneous-sampling ADC with two 4-bit lanes might be
> +wired up like this::
At first, I thought calling these '4-bit lanes' was a bit confusing. I was
thinking about suggesting '4-wire lanes' but I guess 4-bit is more generic in
case we ever see a setup where data navigates through something besides wires.

> +
> +    +--------------+    +----------+
> +    | SPI          |    | AD4630   |
> +    | Controller   |    | ADC      |
> +    |              |    |          |
> +    |          CS0 |--->| CS       |
> +    |          SCK |--->| SCK      |
> +    |          SDO |--->| SDI      |
> +    |              |    |          |
> +    |        SDIA0 |<---| SDOA0    |
> +    |        SDIA1 |<---| SDOA1    |
> +    |        SDIA2 |<---| SDOA2    |
> +    |        SDIA3 |<---| SDOA3    |
> +    |              |    |          |
> +    |        SDIB0 |<---| SDOB0    |
> +    |        SDIB1 |<---| SDOB1    |
> +    |        SDIB2 |<---| SDOB2    |
> +    |        SDIB3 |<---| SDOB3    |
> +    |              |    |          |
> +    +--------------+    +----------+
> +
> +It is described in a devicetree like this::
> +
> +    spi {
> +        compatible = "my,spi-controller";
> +
> +        ...
> +
> +        adc@0 {
> +            compatible = "adi,ad4630";
> +            reg = <0>;
> +            ...
> +            spi-rx-bus-width = <4>, <4>; /* 2 lanes of 4 bits each */
> +            ...
> +        };
> +    };
> +
> +In most cases, lanes will be wired up symmetrically (A to A, B to B, etc). If
> +this isn't the case, extra ``spi-rx-bus-width`` and ``spi-tx-bus-width``
> +properties are needed to provide a mapping between controller lanes and the
> +physical lane wires.
> +
> +Here is an example where a multi-lane SPI controller has each lane wired to
> +separate single-lane peripherals::
> +
> +    +--------------+    +----------+
> +    | SPI          |    | Thing 1  |
> +    | Controller   |    |          |
> +    |              |    |          |
> +    |          CS0 |--->| CS       |
> +    |         SDO0 |--->| SDI      |
> +    |         SDI0 |<---| SDO      |
> +    |        SCLK0 |--->| SCLK     |
> +    |              |    |          |
> +    |              |    +----------+
> +    |              |
> +    |              |    +----------+
> +    |              |    | Thing 2  |
> +    |              |    |          |
> +    |          CS1 |--->| CS       |
> +    |         SDO1 |--->| SDI      |
> +    |         SDI1 |<---| SDO      |
> +    |        SCLK1 |--->| SCLK     |
> +    |              |    |          |
> +    +--------------+    +----------+
> +
> +This is described in a devicetree like this::
> +
> +    spi {
> +        compatible = "my,spi-controller";
> +
> +        ...
> +
> +        thing1@0 {
> +            compatible = "my,thing1";
> +            reg = <0>;
> +            ...
> +        };
> +
> +        thing2@1 {
> +            compatible = "my,thing2";
> +            reg = <1>;
> +            ...
> +            spi-tx-lane-map = <1>; /* lane 0 is not used, lane 1 is used for tx wire */
> +            spi-rx-lane-map = <1>; /* lane 0 is not used, lane 1 is used for rx wire */
In this example, even though lane 0 is not used by thing2, it is being used by
thing1, right?
Just checking I understand it correctly.

> +            ...
> +        };
> +    };
> +