spi: spi-geni-qcom: Actually use our FIFO

spi: spi-geni-qcom: Actually use our FIFO

The geni hardware has a FIFO that can hold up to 64 bytes (it has 16
entries that can hold 4 bytes each), at least on the two SoCs I tested
(sdm845 and sc7180).  We configured our RX Watermark to 0, which
basically meant we got an interrupt as soon as the first 4 bytes
showed up in the FIFO.  Tracing the IRQ handler showed that we often
only read 4 or 8 bytes per IRQ handler.

I tried setting the RX Watermark to "fifo size - 2" but that just got
me a bunch of overrun errors reported.  Setting it to "fifo size - 3"
seemed to work great, though.  This made me worried that we'd start
getting overruns if we had long interrupt latency, but that doesn't
appear to be the case and delays inserted in the IRQ handler while
using "fifo size - 3" didn't cause any errors.  Presumably there is
some interaction with the poorly-documented RFR (ready for receive)
level means that "fifo size - 3" is the max.  We are the SPI master,
so it makes sense that there would be no problems with overruns, the
master should just stop clocking.

Despite "fifo size - 3" working, I chose "fifo size / 2" (8 entries =
32 bytes) which gives us a little extra time to get to the interrupt
handler and should reduce dead time on the SPI wires.  With this
setting, I often saw the IRQ handler handle 40 bytes but sometimes up
to 56 if we had bad interrupt latency.

Testing by running "flashrom -p ec -r" on a Chromebook saw interrupts
from the SPI driver cut roughly in half.  Time was roughly the same.

Fixes: 561de45f72bd ("spi: spi-geni-qcom: Add SPI driver support for GENI based QUP")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20200618080459.v4.4.I988281f7c6ee0ed00325559bfce7539f403da69e@changeid
Signed-off-by: Mark Brown <broonie@kernel.org>