Due to the controller limitation to keep the chip select low during the
bus idle time between the transfer, a dummy cs workaround was used when
this driver was first upstreamed to the kernel.  It basically picks the
dummy cs as !actual_cs so typically dummy cs is 1 when most of the case
only cs 0 is used in the board design. Then invert the polarity of both
cs and tell the controller to start the transfers using dummy cs.
Assuming both cs are active low before the inversion, effectively this
keeps dummy cs high and actual cs low during the transfer and workaround
the issue.

This workaround implies that dummy cs 1 pin has to be set to chip
selection function in the pinmux when the transfer clock is above
25MHz. The old chips likely have default pinmux set to chip select on
the dummy cs pin so it works but this is not case for the new Broadband
BCA chips and this workaround stop working. This is specifically an
issue to support SPI NAND and SPI NOR flash because these flash devices
can typically run at or above 100MHz.

This patch utilizes the prepend feature of the controller to combine the
multiple transfers in the same message to a single transfer when
possible. This way there is no need to keep clock low between transfers
and solve the issue without any hardware requirement.

Multiple transfers within a SPI message may be combined into one
transfer if the following are all true:
  * One or more half duplex write transfer in single bit mode
  * Optional full duplex read/write at the end
  * No delay and cs_change between transfers

Most of the SPI device meets this requirements such as SPI NOR,
SPI NAND flash, Broadcom SPI voice card and etc. For any SPI message
that does not meet the above requirement to combine the transfers, we
switch to original dummy cs mode but limit the clock rate to the safe
25MHz. This is the default auto transfer mode and it makes sure all the
SPI message can be supported automatically under the hood.

This patch also adds the driver sysfs node xfer_mode to provide
the option for overriding the default auto mode and force it to dummy cs
or prepend mode.
Signed-off-by: William Zhang <william.zhang@broadcom.com>
Link: https://lore.kernel.org/r/20230209200246.141520-12-william.zhang@broadcom.comSigned-off-by: Mark Brown <broonie@kernel.org>