[PATCH v2 5/5] ARM: dts: ls1021a-tsn: Use the DSPI controller in poll mode

[Vladimir Oltean <olteanv@gmail.com>]

Connected to the LS1021A DSPI is the SJA1105 DSA switch. This
constitutes 4 of the 6 Ethernet ports on this board.

As the SJA1105 is a PTP switch, constant disciplining of its PTP clock
is necessary, and that translates into a lot of SPI I/O even when
otherwise idle.

Switching to using the DSPI in poll mode has several distinct
benefits:

- With interrupts, the DSPI driver in TCFQ mode raises an IRQ after each
  transmitted byte. There is more time wasted for the "waitq" event than
  for actual I/O. And the DSPI IRQ count is by far the largest in
  /proc/interrupts on this board (larger than Ethernet). I should
  mention that due to various LS1021A errata, other operating modes than
  TCFQ are not available.

- The SPI I/O time is both lower, and more consistently so. For a TSN
  switch it is important that all SPI transfers take a deterministic
  time to complete.
  Reading the PTP clock is an important example.
  Egressing through the switch requires some setup in advance (an SPI
  write command). Without this patch, that operation required a
  --tx_timestamp_timeout 50 (ms), now it can be done with
  --tx_timestamp_timeout 10.
  Yet another example is reconstructing timestamps, which has a hard
  deadline because the PTP timestamping counter wraps around in 0.135
  seconds. Combined with other I/O needed for that to happen, there is
  a real risk that the deadline is not always met.

See drivers/net/dsa/sja1105/ for more info about the above.

Cc: Rob Herring <robh@kernel.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
---
 arch/arm/boot/dts/ls1021a-tsn.dts | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm/boot/dts/ls1021a-tsn.dts b/arch/arm/boot/dts/ls1021a-tsn.dts
index 5b7689094b70..1c09cfc766af 100644
--- a/arch/arm/boot/dts/ls1021a-tsn.dts
+++ b/arch/arm/boot/dts/ls1021a-tsn.dts
@@ -33,6 +33,7 @@
 };
 
 &dspi0 {
+	/delete-property/ interrupts;
 	bus-num = <0>;
 	status = "okay";
 
-- 
2.17.1