linux-kernel - Re: [PATCH 1/3] Input: twl4030-vibra: fix sibling-node lookup

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Date:   Sun, 12 Nov 2017 13:12:35 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Johan Hovold <johan@...nel.org>, linux-input@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable <stable@...r.kernel.org>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        Marek Belisko <marek@...delico.com>,
        Lee Jones <lee.jones@...aro.org>,
        Rob Herring <robh@...nel.org>, devicetree@...r.kernel.org
Subject: Re: [PATCH 1/3] Input: twl4030-vibra: fix sibling-node lookup

[ +CC: Lee, Rob and device-tree list ]

On Sat, Nov 11, 2017 at 09:50:59AM -0800, Dmitry Torokhov wrote:
> On Sat, Nov 11, 2017 at 04:43:37PM +0100, Johan Hovold wrote:
> > A helper purported to look up a child node based on its name was using
> > the wrong of-helper and ended up prematurely freeing the parent of-node
> > while searching the whole device tree depth-first starting at the parent
> > node.
> 
> Ugh, this all is pretty ugly business. Can we teach MFD to allow
> specifying firmware node to be attached to the platform devices it
> creates in mfd_add_device() so that the leaf drivers simply call
> device_property_read_XXX() on their own device and not be bothered with
> weird OF refcount issues or what node they need to locate and parse?

Yeah, that may have helped. You can actually specify a compatible string
in struct mfd_cell today which does make mfd_add_device() associate a
matching child node.

Some best practice regarding how to deal with MFD and device tree would
be good to determine and document too. For example, when should
of_platform_populate() be used in favour of mfd_add_device()?
And how best to deal with sibling nodes, which is part of the problem
here (I think the mfd should have provided a flag rather than having
subdrivers deal with sibling nodes, for example).

That said, driver authors using the wrong of-helper could possibly have
been avoided by amending the kernel docs (I'll do that as a follow up),
but once these incorrect usages get in, only review can prevent them
from being reproduced through copy-paste coding.

Johan