Add a new multi_bus_mode field to struct spi_transfer to allow
peripherals that support multiple SPI buses to be used with a single
SPI controller.

This requires both the peripheral and the controller to have multiple
serializers connected to separate data buses. It could also be used with
a single controller and multiple peripherals that are functioning as a
single logical device (similar to parallel memories).

The possible values for this field have the following semantics:

- SPI_MULTI_BUS_MODE_SINGLE: Only use the first bus. This means that it
    it is operating just like a conventional SPI bus. It is the default
    value so that existing drivers do not need to be modified.

    Example:
        tx_buf[0] = 0x88;

        struct spi_transfer xfer = {
            .tx_buf = tx_buf,
            .len = 1,
        };

        spi_sync_transfer(spi, &xfer, 1);

        controller    > data bits >     peripheral
        ----------   ----------------   ----------
            SDO 0    0-0-0-1-0-0-0-1    SDI 0

- SPI_MULTI_BUS_MODE_MIRROR: Send a single data word over all of the
    buses at the same time. This only makes sense for writes and not
    for reads.

    Example:
        tx_buf[0] = 0x88;

        struct spi_transfer xfer = {
            .tx_buf = tx_buf,
            .len = 1,
            .multi_bus_mode = SPI_MULTI_BUS_MODE_MIRROR,
        };

        spi_sync_transfer(spi, &xfer, 1);

        controller    > data bits >     peripheral
        ----------   ----------------   ----------
            SDO 0    0-0-0-1-0-0-0-1    SDI 0
            SDO 1    0-0-0-1-0-0-0-1    SDI 1

- SPI_MULTI_BUS_MODE_STRIPE: Send or receive two different data words at
    the same time, one on each bus.

    Example:
        struct spi_transfer xfer = {
            .rx_buf = rx_buf,
            .len = 2, /* must be multiple of number of buses */
            .multi_bus_mode = SPI_MULTI_BUS_MODE_STRIPE,
        };

        spi_sync_transfer(spi, &xfer, 1);

        controller    < data bits <     peripheral
        ----------   ----------------   ----------
            SDI 0    0-0-0-1-0-0-0-1    SDO 0
            SDI 1    1-0-0-0-1-0-0-0    SDO 1

        After the transfer, rx_buf[0] == 0x11 (word from SDO 0) and
        rx_buf[1] == 0x88 (word from SDO 1). If the transfer was longer,
        the data would continue in an alternating fashion.

Acked-by: Nuno Sá <nuno.sa@analog.com>
Acked-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
---
 include/linux/spi/spi.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index c314194d4e7e2b396795ece10e142118ca05f4eb..7d22ef5077d1229cac2574831f80092c591b972f 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -976,6 +976,8 @@ struct spi_res {
  *      (SPI_NBITS_SINGLE) is used.
  * @rx_nbits: number of bits used for reading. If 0 the default
  *      (SPI_NBITS_SINGLE) is used.
+ * @multi_bus_mode: How to serialize data on multiple buses. One of the
+ *      SPI_MULTI_BUS_MODE_* values.
  * @len: size of rx and tx buffers (in bytes)
  * @speed_hz: Select a speed other than the device default for this
  *      transfer. If 0 the default (from @spi_device) is used.
@@ -1112,6 +1114,10 @@ struct spi_transfer {
 	unsigned	cs_change:1;
 	unsigned	tx_nbits:4;
 	unsigned	rx_nbits:4;
+	unsigned	multi_bus_mode: 2;
+#define SPI_MULTI_BUS_MODE_SINGLE	0 /* only use single bus */
+#define SPI_MULTI_BUS_MODE_STRIPE	1 /* one data word per bus */
+#define SPI_MULTI_BUS_MODE_MIRROR	2 /* same word sent on all buses */
 	unsigned	timestamped:1;
 	bool		dtr_mode;
 #define	SPI_NBITS_SINGLE	0x01 /* 1-bit transfer */

-- 
2.43.0