spi: Avoid setting the chip select if we don't need to

spi: Avoid setting the chip select if we don't need to

On some SPI controllers (like spi-geni-qcom) setting the chip select
is a heavy operation.  For instance on spi-geni-qcom, with the current
code, is was measured as taking upwards of 20 us.  Even on SPI
controllers that aren't as heavy, setting the chip select is at least
something like a MMIO operation over some peripheral bus which isn't
as fast as a RAM access.

While it would be good to find ways to mitigate problems like this in
the drivers for those SPI controllers, it can also be noted that the
SPI framework could also help out.  Specifically, in some situations,
we can see the SPI framework calling the driver's set_cs() with the
same parameter several times in a row.  This is specifically observed
when looking at the way the Chrome OS EC SPI driver (cros_ec_spi)
works but other drivers likely trip it to some extent.

Let's solve this by caching the chip select state in the core and only
calling into the controller if there was a change.  We check not only
the "enable" state but also the chip select mode (active high or
active low) since controllers may care about both the mode and the
enable flag in their callback.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20200629164103.1.Ied8e8ad8bbb2df7f947e3bc5ea1c315e041785a2@changeid
Signed-off-by: Mark Brown <broonie@kernel.org>