linux-kernel - Re: [PATCH] mfd: mfd-core: Change "Failed to locate of

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Date:   Wed, 30 Jun 2021 13:33:58 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Yunus Bas <Y.Bas@...tec.de>,
        "stwiss.opensource@...semi.com" <stwiss.opensource@...semi.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mfd: mfd-core: Change "Failed to locate of_node" warning
 to debug

On Wed, 30 Jun 2021, Daniel Thompson wrote:

> On Wed, Jun 30, 2021 at 07:27:32AM +0000, Yunus Bas wrote:
> > Am Dienstag, dem 29.06.2021 um 14:39 +0100 schrieb Lee Jones:
> > > On Tue, 29 Jun 2021, Yunus Bas wrote:
> > > > Interestingly, all subdevices defined in the driver are registered
> > > > as platform devices from the MFD framework, regardless of a
> > > > devicetree entry or not. The preceding code checks the subdevice
> > > > cells with an additional compatible. In case a device has no
> > > > devicetree entry, an irritating failed-message is printed on the
> > > > display. I'm not sure if this was the intention but the framework
> > > > somehow forces the users to describe all subdevices of an MFD. I
> > > > think the info print is not needed. It makes more sense to set it
> > > > as a debug print.
> > > 
> > > Actually, this has served to highlight that your DTS is not correct.
> > > 
> > > Why are some devices represented in DT and some aren't?
> > > 
> > > If anything, I'm tempted to upgrade the info() print to warn().
> > 
> > Imagine only required parts of the MFD is connected to the designed
> > system and unrequired parts are not. In that case, fully describing the
> > MFD in the devicetree wouldn't represent the system at all.
> 
> To describe hardware that is present but unused we would normally use
> status = "disabled".
> 
> So if, for example, your board cannot use the RTC for some reason
> (perhaps the board has no 32KHz oscillator?) then the DA9062 still
> contains the hardware but it is useless. Such hardware could be
> described as:
> 
> da9062_rtc: rtc {
>     compatible = "dlg,da9062-rtc";
>     status = "disabled";
> }
> 
> Is this sufficient to suppress the warnings when the hardware is not
> fully described?
> 
> There is almost certainly a problem here since there is a mismatch
> between mfd-core and the DA9062 DT bindings. mfd-core warns when the
> hardware description is incomplete and the DA9062 (and generic mfd) DT
> bindings are ambiguous about whether sub-nodes are mandatory and include
> an example that contains missing compatibles rather than disabled nodes
> like the above.
> 
> However it is not entirely clear to me at this point whether this should
> be fixed in mfd-core or by improving the bindings documentation.

Right.  This is a potential solution.

NB: The suggestion above is usually the default for devices (at least
this was the case back when I was neck deep in DT).  You usually have
the a device specified in a DTSI file with the generic properties
defined from within a top-level node which is usually disabled.  Then
you link back to that node (usually with a &) from within your DTS
file where you provide platform specific properties and override the
status to 'okay' or what have you.

However before I provide any further assistance, I really want to get
an idea of the H/W you're working with.  Is this a reduced function
DA9062?  Or is the functionality actually present, you just don't want
to make use of it?

-- 
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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