On 9/24/20 9:33 AM, Kurt Van Dijck wrote:
>>>> 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.
ACK
I wanted to point out that really using OSC/2 for SPI would be a problem, but
the driver takes care of that now.
>>>> 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.
ACK, I've seen several board that are doing that "mistake".
> Since Matthias uses 'custom iMX6ULL based hardware', I assumed they made
> the same mistake.
probably....
> 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.
Let's say the bits on the SPI bus are transferred twice as fast, the overall
system load isn't.
regards,
Marc
--
Pengutronix e.K. | Marc Kleine-Budde |
Embedded Linux | https://www.pengutronix.de |
Vertretung West/Dortmund | Phone: +49-231-2826-924 |
Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |
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