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902481a78e
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Douglas Anderson 902481a78e
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: 561de45f72 ("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>
2020-06-19 13:25:19 +01:00
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accessibility treewide: replace '---help---' in Kconfig files with 'help' 2020-06-14 01:57:21 +09:00
acpi libnvdimm for 5.8 2020-06-13 13:04:36 -07:00
amba
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base linux-kselftest-kunit-5.8-rc1 2020-06-09 10:04:47 -07:00
bcma
block Kbuild updates for v5.8 (2nd) 2020-06-13 13:29:16 -07:00
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clocksource clocksource/drivers/timer-riscv: Use per-CPU timer interrupt 2020-06-09 19:11:22 -07:00
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counter
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irqchip clocksource/drivers/timer-riscv: Use per-CPU timer interrupt 2020-06-09 19:11:22 -07:00
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mtd This pull request contains a single change for UBI: 2020-06-10 13:24:40 -07:00
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sh
siox
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soundwire
spi spi: spi-geni-qcom: Actually use our FIFO 2020-06-19 13:25:19 +01:00
spmi
ssb
staging Kbuild updates for v5.8 (2nd) 2020-06-13 13:29:16 -07:00
target Kbuild updates for v5.8 (2nd) 2020-06-13 13:29:16 -07:00
tc
tee
thermal - Add the hwmon support on the i.MX SC (Anson Huang) 2020-06-12 14:10:21 -07:00
thunderbolt
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xen The X86 entry, exception and interrupt code rework 2020-06-13 10:05:47 -07:00
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