Messages in this thread | | | Subject | Re: [PATCH 56/62] watchdog: tangox_wdt: Convert to use device managed functions | From | Marc Gonzalez <> | Date | Wed, 11 Jan 2017 16:28:12 +0100 |
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On 11/01/2017 15:25, Guenter Roeck wrote: > On 01/11/2017 04:31 AM, Marc Gonzalez wrote: >> On 11/01/2017 11:52, Guenter Roeck wrote: >> >>> On 01/11/2017 01:07 AM, Marc Gonzalez wrote: >>> >>>>> @@ -134,12 +134,15 @@ static int tangox_wdt_probe(struct platform_device *pdev) >>>>> err = clk_prepare_enable(dev->clk); >>>>> if (err) >>>>> return err; >>>>> + err = devm_add_action_or_reset(&pdev->dev, >>>>> + (void(*)(void *))clk_disable_unprepare, >>>>> + dev->clk); >>>>> + if (err) >>>>> + return err; >>>> >>>> Hello Guenter, >>>> >>>> I would rather avoid the function pointer cast. >>>> How about defining an auxiliary function for the cleanup action? >>>> >>>> clk_disable_unprepare() is static inline, so gcc will have to >>>> define an auxiliary function either way. What do you think? >>> >>> Not really. It would just make it more complicated to replace the >>> call with devm_clk_prepare_enable(), should it ever find its way >>> into the light of day. >> >> More complicated, because the cleanup function will have to be deleted later? >> The compiler will warn if someone forgets to do that. >> >> In my opinion, it's not a good idea to rely on the fact that casting >> void(*)(struct clk *clk) to void(*)(void *) is likely to work as expected >> on most platforms. (It has undefined behavior, strictly speaking.) > > I do hear that you object to this code. > > However, I must admit that you completely lost me here. It is a cast from > one function pointer to another,
Perhaps you are used to work at the assembly level, where pointers are just addresses, and all pointers are interchangeable.
At a slightly higher level (C abstract machine), it is not so.
> passed as argument to another function, > with a secondary cast of its argument from a typed pointer to a void pointer. > I don't think C permits for "undefined behavior, strictly speaking".
The C standard leaves quite a lot of behavior undefined, e.g.
char *foo = "hello"; foo[1] = 'a'; // UB
char buf[4]; *(int *)&buf = 0xdeadbeef; // UB
1 << 64; // UB
> Besides, that same mechanism is already used elsewhere, which is how I > got the idea. Are you claiming that there are situations where it won't > work ?
If this technique is already used elsewhere in the kernel, then I'll crawl back under my rock (and weep).
I can see two issues with the code you propose.
First is the same for all casts: silencing potential warnings, e.g. if the prototype of clk_disable_unprepare ever changed. (Though casts are required for vararg function arguments.)
Second is just theory and not a real-world concern.
>> Do you really dislike the portable solution I suggested? :-( > > It is not more portable than the above. It is more expensive and adds more > code.
Maybe I am mistaken. Can you tell me why adding an auxiliary function is more expensive? (In CPU cycles?)
clk_disable_unprepare() is static inline, so an auxiliary function exists either way (implicit or explicit).
Regards.
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