linux-kernel - Re: [PATCH 1/2] of: Make const the device node pointers in of_clk_get and of_node

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Date:	Sun, 9 Nov 2014 18:50:19 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Jean-Francois Moine <moinejf@...e.fr>
Cc:	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Mark Brown <broonie@...nel.org>,
	Lars-Peter Clausen <lars@...afoo.de>,
	devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] of: Make const the device node pointers in
 of_clk_get and of_node_put

On Sat, Nov 08, 2014 at 07:33:22PM +0100, Jean-Francois Moine wrote:
> Some device nodes are sometimes referenced by const pointers.
> 
> This patch avoids to cast these pointers when calling the functions
> of_clk_get() and of_node_put().

NAK.  Firstly, of_node_put contains a kref, which is /definitely/ not
a const item when kobject_put() is called on it.  Inside the kobject
is a kref, which kobject_put() will call kref_put() on.  kref_put()
in turn will internally call atomic_sub_and_test() on a value embedded
within the kref struct.

Ergo, of_node_put() modifies the struct device_node contents.  Therefore,
of_node_put() definitely not treating the data pointed to as read-only,
and therefore it is completely inappropriate for it to be marked "const".

What this then means is that it fundamentally undermines the idea of
storing the pointer to a device_node as a const pointer, as the device
node must always be modified when you're done with it (because it's a
ref-counted structure.)  So, having it const in your code is a bug.

What this also means is that every other place that you've added const
below is also very dubious.

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