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. -- To unsubscribe from this list: send the line "unsubscribe linux-watchdog" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
