spi: fsl-dspi: avoid SCK glitches with continuous transfers

spi: fsl-dspi: avoid SCK glitches with continuous transfers

The DSPI controller has configurable timing for

(a) tCSC: the interval between the assertion of the chip select and the
    first clock edge

(b) tASC: the interval between the last clock edge and the deassertion
    of the chip select

What is a bit surprising, but is documented in the figure "Example of
continuous transfer (CPHA=1, CONT=1)" in the datasheet, is that when the
chip select stays asserted between multiple TX FIFO writes, the tCSC and
tASC times still apply. With CONT=1, chip select remains asserted, but
SCK takes a break and goes to the idle state for tASC + tCSC ns.

In other words, the default values (of 0 and 0 ns) result in SCK
glitches where the SCK transition to the idle state, as well as the SCK
transition from the idle state, will have no delay in between, and it
may appear that a SCK cycle has simply gone missing. The resulting
timing violation might cause data corruption in many peripherals, as
their chip select is asserted.

The driver has device tree bindings for tCSC ("fsl,spi-cs-sck-delay")
and tASC ("fsl,spi-sck-cs-delay"), but these are only specified to apply
when the chip select toggles in the first place, and this timing
characteristic depends on each peripheral. Many peripherals do not have
explicit timing requirements, so many device trees do not have these
properties present at all.

Nonetheless, the lack of SCK glitches is a common sense requirement, and
since the SCK stays in the idle state during transfers for tCSC+tASC ns,
and that in itself should look like half a cycle, then let's ensure that
tCSC and tASC are at least a quarter of a SCK period, such that their
sum is at least half of one.

Fixes: 95bf15f38641 ("spi: fsl-dspi: Add ~50ns delay between cs and sck")
Reported-by: Lisa Chen (陈敏捷) <minjie.chen@geekplus.com>
Debugged-by: Lisa Chen (陈敏捷) <minjie.chen@geekplus.com>
Tested-by: Lisa Chen (陈敏捷) <minjie.chen@geekplus.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20230529223402.1199503-1-vladimir.oltean@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>