Re: [PATCH v5 3/9] spi: support controllers with multiple data lanes

[Jonathan Cameron <jic23@kernel.org>]

On Mon, 12 Jan 2026 11:45:21 -0600
David Lechner <dlechner@baylibre.com> wrote:

> Add support for SPI controllers with multiple physical SPI data lanes.
> (A data lane in this context means lines connected to a serializer, so a
> controller with two data lanes would have two serializers in a single
> controller).
> 
> This is common in the type of controller that can be used with parallel
> flash memories, but can be used for general purpose SPI as well.
> 
> To indicate support, a controller just needs to set ctlr->num_data_lanes
> to something greater than 1. Peripherals indicate which lane they are
> connected to via device tree (ACPI support can be added if needed).
> 
> The spi-{tx,rx}-bus-width DT properties can now be arrays. The length of
> the array indicates the number of data lanes, and each element indicates
> the bus width of that lane. For now, we restrict all lanes to have the
> same bus width to keep things simple. Support for an optional controller
> lane mapping property is also implemented.
> 
> Signed-off-by: David Lechner <dlechner@baylibre.com>
> ---
> 
> v5 changes:
> - Use of_property_read_variable_u32_array() for lane maps.
For this, I think you need to check for short maps.
> 
> v4 changes:
> - Update for changes in devicetree bindings.
> - Don't put new fields in the middle of CS fields.
> - Dropped acks since this was a significant rework.
> 
> v3 changes:
> * Renamed "buses" to "lanes" to reflect devicetree property name change.
> 
> This patch has been seen in a different series [1] by Sean before:
> 
> [1]: https://lore.kernel.org/linux-spi/20250616220054.3968946-4-sean.anderson@linux.dev/
> 
> Changes:
> * Use u8 array instead of bitfield so that the order of the mapping is
>   preserved. (Now looks very much like chip select mapping.)
> * Added doc strings for added fields.
> * Tweaked the comments.
> ---
>  drivers/spi/spi.c       | 116 +++++++++++++++++++++++++++++++++++++++++++++++-
>  include/linux/spi/spi.h |  22 +++++++++
>  2 files changed, 136 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
> index e25df9990f82..5c3f9ba3f606 100644
> --- a/drivers/spi/spi.c
> +++ b/drivers/spi/spi.c
> @@ -2370,7 +2370,53 @@ static int of_spi_parse_dt(struct spi_controller *ctlr, struct spi_device *spi,
>  		spi->mode |= SPI_CS_HIGH;
>  
>  	/* Device DUAL/QUAD mode */
> -	if (!of_property_read_u32(nc, "spi-tx-bus-width", &value)) {
> +
> +	rc = of_property_read_variable_u32_array(nc, "spi-tx-lane-map",
> +						 spi->tx_lane_map, 1,
> +						 ARRAY_SIZE(spi->tx_lane_map));

This reads 'up to' the ARRAY_SIZE(spi->tx_lane_map)
If it is short, what is the right thing to do?  I'd either expect a check
for that or for rc to be stashed somewhere if positive for later use.
If the intent is for short the default of 0 is fine, then if it's a lot
short we'll end up with repeated mappings to 0 which makes little sense.


> +	if (rc == -EINVAL) {
> +		/* Default lane map */
> +		for (idx = 0; idx < ARRAY_SIZE(spi->tx_lane_map); idx++)
> +			spi->tx_lane_map[idx] = idx;
> +	} else if (rc < 0) {
> +		dev_err(&ctlr->dev,
> +			"failed to read spi-tx-lane-map property: %d\n", rc);
> +		return rc;
> +	}
> +
> +	rc = of_property_count_u32_elems(nc, "spi-tx-bus-width");
> +	if (rc < 0 && rc != -EINVAL) {
> +		dev_err(&ctlr->dev,
> +			"failed to read spi-tx-bus-width property: %d\n", rc);
> +		return rc;
> +	}
> +
> +	if (rc == -EINVAL) {
> +		/* Default when property is not present. */
> +		spi->num_tx_lanes = 1;
> +	} else {
> +		u32 first_value;
> +
> +		spi->num_tx_lanes = rc;
> +
> +		for (idx = 0; idx < spi->num_tx_lanes; idx++) {
> +			of_property_read_u32_index(nc, "spi-tx-bus-width", idx,
> +						   &value);

Probably want a sanity check on return value of that even though we are fairly sure
it won't fail.

> +
> +			/*
> +			 * For now, we only support all lanes having the same
> +			 * width so we can keep using the existing mode flags.
> +			 */
> +			if (!idx)
> +				first_value = value;
> +			else if (first_value != value) {
> +				dev_err(&ctlr->dev,
> +					"spi-tx-bus-width has inconsistent values: first %d vs later %d\n",
> +					first_value, value);
> +				return -EINVAL;
> +			}
> +		}