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    SubjectRe: [PATCH 56/62] watchdog: tangox_wdt: Convert to use device managed functions
    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).


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