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    SubjectRe: Inconsistency in clk framework
    On Thu, Dec 20, 2012 at 05:13:37PM +1300, Tony Prisk wrote:
    > On Wed, 2012-12-19 at 19:08 +0000, Russell King - ARM Linux wrote:
    > > On Thu, Dec 20, 2012 at 08:00:49AM +1300, Tony Prisk wrote:
    > > > On Thu, 2012-12-20 at 06:34 +1300, Tony Prisk wrote:
    > > > > On Wed, 2012-12-19 at 09:26 +0000, Russell King - ARM Linux wrote:
    > > > > > On Wed, Dec 19, 2012 at 05:10:33PM +1300, Tony Prisk wrote:
    > > > > > > Hi Mike,
    > > > > > >
    > > > > > > In attempting to remove some IS_ERR_OR_NULL references, it was pointed
    > > > > > > out that clk_get() can return NULL if CONFIG_HAVE_CLK is not defined.
    > > > > >
    > > > > > That is correct - but why is that a problem? As far as users are
    > > > > > concerned, NULL is a valid clock. If HAVE_CLK is undefined, do you
    > > > > > want all your drivers to suddenly stop working?
    > > > >
    > > > > That will be where the misunderstanding has occurred - I didn't consider
    > > > > NULL to be a valid clock.
    > > > >
    > > > > Given that NULL is a valid clock, I guess all tests against get_clk and
    > > > > of_get_clk results should be IS_ERR_OR_NULL. Correct?
    > > > >
    > > > For the sake of clarity, I should rephrase:
    > > >
    > > > 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.
    > >
    > > 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.
    >
    > As a simple example:
    > We have a PWM module that requires a clock source to be enabled before
    > registers can be read/written.
    >
    > *pseudo code*
    > x = clk_get("pwm_clock")
    > if IS_ERR(x) then fail
    > err = clk_enable(x)
    > if (err != 0) then fail
    > start writing to module registers
    >
    >
    > Assuming HAVE_CLK is undefined:
    >
    > x = clk_get("pwm_clock") (= NULL)
    > if IS_ERR(x) then fail (not an error)
    > err = clk_enable(x) (= 0)
    > if (err) then fail (not an error)
    > start writing to module registers
    > (register writes lock the bus because the clock wasn't really enabled,
    > but no errors occurred enabling the clock)

    Which is really silly if you have a platform which requires clock control
    and HAVE_CLK is not selected. The clk API is not designed to cope with
    that situation.


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