spi: Add support for Double Transfer Rate (DTR) mode

Introduce support for protocol drivers to specify whether a transfer
should use single or dual transfer mode. Currently, the SPI controller
cannot determine this information from the user, leading to potential
limitations in transfer capabilities.

Add a new field `dtr_mode` in the `spi_transfer` structure. The `dtr_mode`
field allows protocol drivers to indicate if Double Transfer Rate (DTR)
mode is supported for a given transfer. When `dtr_mode` is set to true,
the SPI controller will use DTR mode; otherwise, it will default to single
transfer mode.

Introduce another field `dtr_caps` to indicate if the QSPI controller is
capable of supporting DTR mode (SDR and DDR). By default, both `dtr_caps`
and `dtr_mode` will be false. These flags manage the QSPI controller's DTR
mode capabilities within the SPI framework.

The QSPI controller driver uses these flags to configure single or double
transfer rates using the controller register.

The existing spi-mem driver helps configure the DTR mode but is limited to
memory devices. There is no support available to set DTR mode for non-memory
devices, e.g., touch or any generic SPI sensor. This change is backward
compatible and doesn't break existing SPI or QSPI drivers.

Changes include:
- Addition of `dtr_mode` and `dtr_caps` fields in the `spi_transfer`
  structure.
- Documentation updates to reflect the new `dtr_mode` and `dtr_caps` fields.

Signed-off-by: Mukesh Kumar Savaliya <quic_msavaliy@quicinc.com>
Link: https://patch.msgid.link/20250404135427.313825-1-quic_msavaliy@quicinc.com
Signed-off-by: Mark Brown <broonie@kernel.org>

0 files changed, 0 insertions, 0 deletions