Hi Jörg, all,
On 05/01/2025 18:08, Andrew Lunn wrote:
On Sun, Jan 05, 2025 at 05:33:38PM +0100, Jörg Sommer wrote:
Hi everyone,
I've added you to the list of recipients, because you where somehow involved
in changes of the KSZ-SPI switch code.
We are debating the SPI mode setting for the microchip ksz8795 and ksz9477
and possibly others. Since the commit
8c4599f49841dd663402ec52325dc2233add1d32 the SPI mode gets fixed to mode 3
in the code. But at least my ksz8795 works also with mode 0 and shows better
initialization behaviour with mode 0.
The big question is: can both (or all ksz) chips work with both modes?
Should this setting stay in code or moved to the device tree?
The specs are here, but I found no evidence about the supported/recommended
SPI modes:
https://ww1.microchip.com/downloads/en/DeviceDoc/KSZ9563R-Data-Sheet-DS00002419D.pdf
From this KSZ9563 datasheet it is quite clear from Figure 6-9 that it
requires mode 0, for KSZ8794 (which I have and can test) Figure 7-8 [1]
also indicates mode 0. Note however that older KSZ8794 datasheets
(revision DS00002134A from 2016, can upload if needed) rather indicate a
mode 3, which is a hint to me that indeed both modes were once
supported. Appendix A from [1] states that in 2021 the SPI Timing images
and parameters have been updated. No further information there but your
experience and the datasheet update seem to indicate mode 0 has better
support.
My SPI peripheral (and KSZ8 driver) is configured for mode 3 I see but
the frequency is set to 10 MHz while the max is 50 MHz. From experience
with other SPI devices I know that with higher frequencies the timing
parameters (setup and hold times, mode) become more important, what
frequency do you have configured Jörg? I'm asking because I don't
experience the issue you have.
[1]
https://ww1.microchip.com/downloads/aemDocuments/documents/UNG/ProductDocuments/DataSheets/KSZ8794CNX-Data-Sheet-DS00002134.pdf
My interpretation of this is that the initial clock priority is low,
so CPOL=0. The rising edge will be the leading edge, so CPHA=0. So
that makes the mode = 0.
Cheers, Pieter
