Re: [PATCH 2/6] spi: add per-transfer CS mask

[Nuno Sá]

On Tue, Jul 14, 2026 at 02:56:02AM -0300, Jonathan Santos wrote:
> Multi-CS devices assert all their chip selects simultaneously by default
> via cs_index_mask. Some use cases require selecting a subset of those CS
> lines per transfer (e.g. routing data to one of several subdevices sharing
> the bus).
> 
> Add a cs_select_mask field to spi_transfer. When the mask is set and the
> controller has SPI_CONTROLLER_MULTI_CS, spi_set_cs() builds a temporary
> spi_device whose chip_select[] and cs_gpiod[] arrays contain only the lines
> indicated by the mask, then delegates to the renamed _spi_set_cs().
> When cs_select_mask is 0 or the controller does not support multi-CS,
> the device default (cs_index_mask) is used unchanged.
> 
> Update spi_transfer_one_message() to pass the relevant spi_transfer to
> every spi_set_cs() call so CS transitions between transfers with
> different masks are handled correctly.
> 
> Signed-off-by: Jonathan Santos <Jonathan.Santos@xxxxxxxxxx>
> ---
>  drivers/spi/spi.c       | 88 +++++++++++++++++++++++++++++++++++------
>  include/linux/spi/spi.h |  4 ++
>  2 files changed, 81 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
> index 55fb96fea243..6d644adf8445 100644
> --- a/drivers/spi/spi.c
> +++ b/drivers/spi/spi.c
> @@ -1071,7 +1071,7 @@ static void spi_toggle_csgpiod(struct spi_device *spi, u8 idx, bool enable, bool
>  		spi_delay_exec(&spi->cs_inactive, NULL);
>  }
>  
> -static void spi_set_cs(struct spi_device *spi, bool enable, bool force)

Might be missing something but spi_set_cs() in the current code does:

spi_get_chipselect(spi, 0);

And I'm not really seeing nothing in the patch handling it. Is that not
an issue?

> +static void _spi_set_cs(struct spi_device *spi, bool enable, bool force)
>  {
>  	bool activate = enable;
>  	u8 idx;
> @@ -1129,6 +1129,61 @@ static void spi_set_cs(struct spi_device *spi, bool enable, bool force)
>  	}
>  }
>  
> +static void spi_set_cs(struct spi_device *spi,
> +		       struct spi_transfer *xfer,
> +		       bool enable, bool force)
> +{
> +	struct spi_controller *ctlr = spi->controller;
> +	struct spi_device temp_spi;

Not sure what to feel about the above temp_spi dance. Can't we just pass a cs_mask
into _spi_set_cs() and change the code accordingly? And for the default
case we would just pass spi->cs_index_mask.

- Nuno Sá