linux-kernel - Re: [PATCH 3/6] spi: add multi_bus_mode field to struct spi

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Message-ID: <9269eadc1ea593e5bc8f5cad8061b48220f4d2b2.camel@gmail.com>
Date: Wed, 15 Oct 2025 11:16:01 +0100
From: Nuno Sá <noname.nuno@...il.com>
To: David Lechner <dlechner@...libre.com>, Mark Brown <broonie@...nel.org>, 
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>,  Marcelo Schmitt
 <marcelo.schmitt@...log.com>, Michael Hennerich
 <michael.hennerich@...log.com>, Nuno Sá	
 <nuno.sa@...log.com>, Jonathan Cameron <jic23@...nel.org>, Andy Shevchenko	
 <andy@...nel.org>
Cc: Sean Anderson <sean.anderson@...ux.dev>, linux-spi@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-iio@...r.kernel.org
Subject: Re: [PATCH 3/6] spi: add multi_bus_mode field to struct spi_transfer

On Tue, 2025-10-14 at 17:02 -0500, David Lechner wrote:
> 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

Out of curiosity, how does this work for devices like AD4030 where the same word
is kind of interleaved between SDO lines? I guess it works the same (in terms of
SW) and is up to some IP core (typically in the FPGA) to "re-assemble" the word?

> 
>         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.
> 
> Signed-off-by: David Lechner <dlechner@...libre.com>
> ---

Looks good anyways,

Acked-by: Nuno Sá <nuno.sa@...log.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..7d22ef5077d1229cac2574831f80092c591b
> 972f 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 */