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

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Date:   Fri, 2 Jul 2021 19:36:07 +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 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.

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