linux-kernel - Re: [PATCH RESEND 6/6] clk: s5p-g2d: Fix incorrect usage of IS_ERR_OR

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Message-ID: <20130103100027.GK2631@n2100.arm.linux.org.uk>
Date:	Thu, 3 Jan 2013 10:00:27 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Julia Lawall <julia.lawall@...6.fr>
Cc:	Dan Carpenter <dan.carpenter@...cle.com>,
	Tomasz Stanislawski <t.stanislaws@...sung.com>,
	Dan Carpenter <error27@...il.com>,
	Sergei Shtylyov <sshtylyov@...sta.com>,
	kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Sylwester Nawrocki <sylvester.nawrocki@...il.com>,
	linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org
Subject: Re: [PATCH RESEND 6/6] clk: s5p-g2d: Fix incorrect usage of
	IS_ERR_OR_NULL

On Thu, Jan 03, 2013 at 10:14:13AM +0100, Julia Lawall wrote:
> On Thu, 3 Jan 2013, Dan Carpenter wrote:
> 
> > On Wed, Jan 02, 2013 at 06:31:53PM +1300, Tony Prisk wrote:
> > > On Wed, 2013-01-02 at 08:10 +0300, Dan Carpenter wrote:
> > > > clk_get() returns NULL if CONFIG_HAVE_CLK is disabled.
> > > >
> > > > I told Tony about this but everyone has been gone with end of year
> > > > holidays so it hasn't been addressed.
> > > >
> > > > Tony, please fix it so people don't apply these patches until
> > > > clk_get() is updated to not return NULL.  It sucks to have to revert
> > > > patches.
> > > >
> > > > regards,
> > > > dan carpenter
> > >
> > > I posted the query to Mike Turquette, linux-kernel and linux-arm-kernel
> > > mailing lists, regarding the return of NULL when HAVE_CLK is undefined.
> > >
> > > Short Answer: A return value of NULL is valid and not an error therefore
> > > we should be using IS_ERR, not IS_ERR_OR_NULL on clk_get results.
> > >
> > > I see the obvious problem this creates, and asked this question:
> > >
> > > If the driver can't operate with a NULL clk, it should use a
> > > IS_ERR_OR_NULL test to test for failure, rather than IS_ERR.
> > >
> > >
> > > And Russell's answer:
> > >
> > > Why should a _consumer_ of a clock care?  It is _very_ important that
> > > people get this idea - to a consumer, the struct clk is just an opaque
> > > cookie.  The fact that it appears to be a pointer does _not_ mean that
> > > the driver can do any kind of dereferencing on that pointer - it should
> > > never do so.
> > >
> > > Thread can be viewed here:
> > > https://lkml.org/lkml/2012/12/20/105
> > >
> >
> > Ah.  Grand.  Thanks...
> >
> > Btw. The documentation for clk_get() really should include some of
> > this information.  I know Russell thinks that the driver authors are
> > stupid and lazy, and it's probably true.  But if everyone makes the
> > same mistake over and over, then it probably means we could put a
> > special note:
> >
> > "Do not check this with IS_ERR_OR_NULL().  Null values are not an
> > error.  Drivers should treat the return value as an opaque cookie
> > and they should not dereference it."
> >
> > This is probably there in the file somewhere else, but I searched
> > for "opaque", "cookie", and "dereference" and I didn't find
> > anything.  I'm not saying the documentation isn't perfect, just that
> > driver authors are lazy and stupid but we can't kill them so we have
> > to live with them.
> 
> I still think it would also be helpful for the definition that returns
> NULL to have some documentation associated with it.  Having a feature
> disabled and then trying to use the feature could reasonably considered to
> lead to a failure, so it is not obvious what the NULL represents.

/**
 * clk_get - lookup and obtain a reference to a clock producer.
 * @dev: device for clock "consumer"
 * @id: clock consumer ID
 *
 * Returns a struct clk corresponding to the clock producer, or
 * valid IS_ERR() condition containing errno.  The implementation
 * uses @dev and @id to determine the clock consumer, and thereby
 * the clock producer.  (IOW, @id may be identical strings, but
 * clk_get may return different clock producers depending on @dev.)
 *
 * Drivers must assume that the clock source is not enabled.
 *
 * clk_get should not be called from within interrupt context.
 */

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