On do, 24 sep 2020 08:50:45 +0200, Marc Kleine-Budde wrote:
> On 9/23/20 9:05 PM, Kurt Van Dijck wrote:
> > On wo, 23 sep 2020 16:55:13 +0200, Matthias Weißer wrote:
> >> Hi
> >>
> >> I currently try to get a MCP2518FD to work on our custom iMX6ULL based
> >> hardware. I use the driver currently in linux-can-next backported to
> >> our v5.4 kernel
> >>
> >> DT:
> >> can0: can@0 {
> >> compatible = "microchip,mcp25xxfd";
> >> reg = <0>;
> >> clocks = <&can3_osc>;
> >> spi-max-frequency = <85000000>;
>
> > I did not experience issues by using the real 20MHz upper limit here.
>
> Not quite...
Well, the driver calculated things correctly, and it chose some 8.xx MHz
to use. I didn't need to limit the spi-max-frequency manually.
>
> >> interrupts-extended = <&gpio1 18 IRQ_TYPE_LEVEL_LOW>;
> >> status = "okay";
> >> };
> >>
> >> # dmesg | grep mcp
> >> [ 3.706085] mcp25xxfd spi1.0 can0: MCP2518FD rev0.0 (-RX_INT
> >> -MAB_NO_WARN +CRC_REG +CRC_RX +CRC_TX +ECC -HD c:20.00MHz m:8.50MHz
> >> r:8.50MHz e:0.00MHz) successfully initialized.
> >
> > off-topic:
> > Variscite deploys a 20HHz oscillator, which brings the max spi frequency
> > down to +/- 8.5MHz.
> > After discussion with Thomas Kopp, I learned that microchip uses a 40MHz
> > oscillator, to arrive (close to) 20MHz spi freq.
> > Would you not use a 40MHz oscillator too?
>
> According to the debug output the can3_osc is 20 MHz. According to the datasheet
> this makes 10 MHz SPI clock, however it turns out, you can only use 85% of that
> to avoid stability problems. The driver takes care of the 85%, so you don't have
> to specify this in the DT.
I would have liked if Variscite had better investigated how to provide
the best performance using the chip. Instead, someone read '20MHz max spi frequency',
and put a 20MHz oscillator on the board.
Since Matthias uses 'custom iMX6ULL based hardware', I assumed they made
the same mistake.
my point here was that they could supply a 40MHz clock, and the driver would then
derive the spi max frequency to 0.85*20MHz = 17MHz. Things goes twice as
fast then.
>
> Marc
>
> --
> Pengutronix e.K. | Marc Kleine-Budde |
> Embedded Linux | https://www.pengutronix.de |
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>
