netdev - Re: KSZ8795 not detected at start to boot from NFS

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Message-ID: <3e7264a5-aea7-44f1-8a37-2b8cf48b710c@cern.ch>
Date: Mon, 6 Jan 2025 14:38:02 +0100
From: Pieter Van Trappen <pieter.van.trappen@...n.ch>
To: Jörg Sommer <joerg@...so.de>
Cc: Christian Eggers <ceggers@...i.de>, Jakub Kicinski <kuba@...nel.org>,
 Tristram Ha <tristram.ha@...rochip.com>,
 Woojung Huh <Woojung.Huh@...rochip.com>, netdev@...r.kernel.org,
 linux-spi@...r.kernel.org, Andrew Lunn <andrew@...n.ch>
Subject: Re: KSZ8795 not detected at start to boot from NFS

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