Hi Mark,
On Fri, 23 Aug 2019 at 00:15, Vladimir Oltean <olteanv@xxxxxxxxx> 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 <robh@xxxxxxxxxx>
> Cc: Shawn Guo <shawnguo@xxxxxxxxxx>
> Signed-off-by: Vladimir Oltean <olteanv@xxxxxxxxx>
> ---
> 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
