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

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Message-ID: <20210702191012.mecgw577ggkabxr6@maple.lan>
Date:   Fri, 2 Jul 2021 20:10:12 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Lee Jones <lee.jones@...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 Fri, Jul 02, 2021 at 07:36:07PM +0100, Lee Jones wrote:
> On Fri, 02 Jul 2021, Daniel Thompson wrote:
> 
> > On Thu, Jul 01, 2021 at 03:34:43PM +0000, Yunus Bas wrote:
> > > Am Mittwoch, dem 30.06.2021 um 13:33 +0100 schrieb Lee Jones:
> > > > 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:
> > > > > > 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?
> > <snip>
> > > > 
> > > > Right.  This is a potential solution.
> > > 
> > > @Daniel, you hit the nail on the head :). Thank you for that.
> > > 
> > > This solution would indeed surpress the warnings, but what is the
> > > benefit of this? We would define never used device nodes just to
> > > satisfy the driver.
> > 
> > I would say that doing so resolves an awkward ambiguity of
> > interpretation w.r.t. the bindings.
> > 
> > 1. The MFD device compatible "dlg,da9062" tells the OS that we
> >    have an DA9062. An DA9062 contains six functions and this can be
> >    inferred *entirely* from the MFD compatible string. We do not
> >    need any subnodes to tell us that a DA9062 contains an RTC. The OS
> >    can (and in this case, does) already know that there is an RTC
> >    because we have a DA9062 (and a datasheet).
> > 
> > 2. The default behaviour when a node has no status field is to
> >    assume that is is *enabled*.
> > 
> > Based on #1 and #2 above then assuming that a DT that omits the
> > sub-nodes actually means "disable the RTC" is risky. #2 might
> > actually make it more natural to assume that the device is present and
> > functional because there is no status field to tell MFD *not* to
> > initialize it.
> 
> Exactly.  Nicely put.
> 
> > That leaves us in a situation where there is no way to correctly guess
> > the authors intent when sub-nodes are omitted from the DT.
> 
> > Given this is something of a corner case and the documentation is
> > ambiguous then a warning of the author does not clearly resolve the
> > ambiguity seems reasonable.
> 
> I'm having trouble parsing this part.

That's quite reasonable because was is written is nonsense!
Perhaps s/warning of the author/warning if the author/ will help
but there are still too many words to say something very simple.
The whole last paragraph could simply say:

  The bindings documentation is ambiguous so is it reasonable
  for the OS to issue a warning when the devicetree author does
  not clearly resolve the ambiguity.

This is still a long sentence but at least it is no longer a
complicated one!


Daniel.