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

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Date:	Wed, 02 Jan 2013 18:31:53 +1300
From:	Tony Prisk <linux@...sktech.co.nz>
To:	Dan Carpenter <error27@...il.com>
Cc:	Sylwester Nawrocki <sylvester.nawrocki@...il.com>,
	Sergei Shtylyov <sshtylyov@...sta.com>,
	kernel-janitors@...r.kernel.org,
	Tomasz Stanislawski <t.stanislaws@...sung.com>,
	linux-kernel@...r.kernel.org,
	Kyungmin Park <kyungmin.park@...sung.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 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

Regards
Tony Prisk

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