Messages in this thread | | | Subject | Re: Inconsistency in clk framework | From | Tony Prisk <> | Date | Thu, 20 Dec 2012 17:13:37 +1300 |
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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)
I apologise if it seems like I am not getting it, but I would like to understand this properly to avoid further problems later.
Regards Tony P
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