[PATCH V5 09/15] spi: Add stacked and parallel memories support in SPI core

[Jonas Gorski]

Hi,

On Mon, 6 Mar 2023 at 18:26, Amit Kumar Mahapatra
<amit.kumar-mahapatra at amd.com> wrote:
>
> For supporting multiple CS the SPI device need to be aware of all the CS
> values. So, the "chip_select" member in the spi_device structure is now an
> array that holds all the CS values.
>
> spi_device structure now has a "cs_index_mask" member. This acts as an
> index to the chip_select array. If nth bit of spi->cs_index_mask is set
> then the driver would assert spi->chip_select[n].
>
> In parallel mode all the chip selects are asserted/de-asserted
> simultaneously and each byte of data is stored in both devices, the even
> bits in one, the odd bits in the other. The split is automatically handled
> by the GQSPI controller. The GQSPI controller supports a maximum of two
> flashes connected in parallel mode. A "multi-cs-cap" flag is added in the
> spi controntroller data, through ctlr->multi-cs-cap the spi core will make
> sure that the controller is capable of handling multiple chip selects at
> once.
>
> For supporting multiple CS via GPIO the cs_gpiod member of the spi_device
> structure is now an array that holds the gpio descriptor for each
> chipselect.
>
> Multi CS support using GPIO is not tested due to unavailability of
> necessary hardware setup.
>
> Signed-off-by: Amit Kumar Mahapatra <amit.kumar-mahapatra at amd.com>
> ---
>  drivers/spi/spi.c       | 213 +++++++++++++++++++++++++++-------------
>  include/linux/spi/spi.h |  34 +++++--
>  2 files changed, 173 insertions(+), 74 deletions(-)
>
> diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
> index 5866bf5813a4..8ec7f58fa111 100644
> --- a/drivers/spi/spi.c
> +++ b/drivers/spi/spi.c
> @@ -613,7 +613,8 @@ static int spi_dev_check(struct device *dev, void *data)
>         struct spi_device *new_spi = data;
>
>         if (spi->controller == new_spi->controller &&
> -           spi_get_chipselect(spi, 0) == spi_get_chipselect(new_spi, 0))
> +           spi_get_chipselect(spi, 0) == spi_get_chipselect(new_spi, 0) &&
> +           spi_get_chipselect(spi, 1) == spi_get_chipselect(new_spi, 1))
>                 return -EBUSY;

This will only reject new devices if both chip selects are identical,
but not if they only share one, e.g. CS 1 + 2 vs 1 + 3, or 1 + 2 vs
only 2, or if the order is different (1 + 2 vs 2 + 1 - haven't read
the code too close to know if this is allowed/possible).

Regards,
Jonas