linux-kernel - Re: [PATCH v2 2/2] net: phy: dp83826: support TX data voltage tuning

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Date: Thu, 8 Feb 2024 14:56:33 +0100
From: Andrew Lunn <andrew@...n.ch>
To: POPESCU Catalin <catalin.popescu@...ca-geosystems.com>
Cc: "davem@...emloft.net" <davem@...emloft.net>,
	"kuba@...nel.org" <kuba@...nel.org>,
	"pabeni@...hat.com" <pabeni@...hat.com>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	"krzysztof.kozlowski+dt@...aro.org" <krzysztof.kozlowski+dt@...aro.org>,
	"conor+dt@...nel.org" <conor+dt@...nel.org>,
	"afd@...com" <afd@...com>,
	"hkallweit1@...il.com" <hkallweit1@...il.com>,
	"linux@...linux.org.uk" <linux@...linux.org.uk>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	GEO-CHHER-bsp-development <bsp-development.geo@...ca-geosystems.com>,
	"m.felsch@...gutronix.de" <m.felsch@...gutronix.de>
Subject: Re: [PATCH v2 2/2] net: phy: dp83826: support TX data voltage tuning

> > I could be reading this wrong, but it looks like
> > DP83826_CFG_DAC_MINUS_DEFAULT actually means leave the value
> > unchanged? Is there anything guaranteeing it does in fact have the
> > default value in the hardware?
> >
> >          Andrew
> 
> Yes, the datasheet clearly states the default/reset values of both 
> registers VOD_CFG1 & VOD_CFG2 which are :
> - cfg_dac_minus : 30h
> - cfg_dac_plus : 10h

And the device is actually and always reset by Linux when the driver
loads? Anything the bootloader has done, or a previous kernel, will be
cleared?

Please add this explanation to the commit message.

I'm being pedantic because we have had problems like this in the past.
If a register was not actually set back to the default value, the
bootloader set it to some other value, the board can work fine. Then a
board can came along which the bootloader set the wrong value, and the
default is actually needed. Fixing the driver to actually enforce the
default breaks boards...

	 Andrew