Hi Mark, On Fri, 23 Aug 2019 at 00:15, Vladimir Oltean <olte...@gmail.com> wrote: > > 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 <r...@kernel.org> > Cc: Shawn Guo <shawn...@kernel.org> > Signed-off-by: Vladimir Oltean <olte...@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 >
I noticed you skipped applying this patch, and I'm not sure that Shawn will review it/take it. Do you have a better suggestion how I can achieve putting the DSPI driver in poll mode for this board? A Kconfig option maybe? Regards, -Vladimir