linux-kernel - Re: [PATCH 4/9v2] usb: host: ehci.h: fix single statement macros

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Date:	Thu, 10 Dec 2015 20:47:05 +0300
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	"Geyslan G. Bem" <geyslan@...il.com>
Cc:	Peter Senna Tschudin <peter.senna@...il.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-usb@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/9v2] usb: host: ehci.h: fix single statement macros

On 12/10/2015 08:40 PM, Geyslan G. Bem wrote:

>>>>>>>> Don't use the 'do {} while (0)' wrapper in a single statement macro.
>>>>>>>>
>>>>>>>> Caught by checkpatch: "WARNING: Single statement macros should not
>>>>>>>> use a do {} while (0) loop"
>>>>>>>>
>>>>>>>> Signed-off-by: Geyslan G. Bem <geyslan@...il.com>
>>>>>>>> ---
>>>>>>>>      drivers/usb/host/ehci.h | 4 ++--
>>>>>>>>      1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/drivers/usb/host/ehci.h b/drivers/usb/host/ehci.h
>>>>>>>> index cfeebd8..945000a 100644
>>>>>>>> --- a/drivers/usb/host/ehci.h
>>>>>>>> +++ b/drivers/usb/host/ehci.h
>>>>>>>> @@ -244,9 +244,9 @@ struct ehci_hcd {                   /* one per
>>>>>>>> controller */
>>>>>>>>            /* irq statistics */
>>>>>>>>      #ifdef EHCI_STATS
>>>>>>>>            struct ehci_stats       stats;
>>>>>>>> -#      define COUNT(x) do { (x)++; } while (0)
>>>>>>>> +#      define COUNT(x) ((x)++)
>>>>>>>>      #else
>>>>>>>> -#      define COUNT(x) do {} while (0)
>>>>>>>> +#      define COUNT(x) ((void) 0)
>>>>>>>
>>>>>>>       Why not just empty #define?
>>>>>>
>>>>>> Indeed. I'll change it.
>>>>>> Tks Sergei.
>>>>>
>>>>> Since COUNT is not used to return the empty #define is ok. Another way
>>>>> is to use #define COUNT(x) (0) to get a 0 when necessary to read
>>>>> returns.
>>
>>>>      Just 0, no parens please.
>>
>>> Ok, no parens, since there's no evaluation.
>>
>>     It's because the literals don't need parens at all.
>>
>>> Then my change is:
>>>
>>> -#      define COUNT(x) do { (x)++; } while (0)
>>> +#      define COUNT(x) (++(x))
>>>    #else
>>> -#      define COUNT(x) do {} while (0)
>>> +#      define COUNT(x) 0
>>>
>>> Pre-increment allowing to return the updated x.
>>
>>
>>     Why if there was a post-increment before?

> There's nothing wrong with post-increment. The pre one would be
> necessary if using return.

    Maybe it was intended to return the old value? :-)

>>
>>     Anyway, this talk is quite pointless since the macro didn't return any
>> value anyway.
> You're sure, there's no use anywhere of the return of that macro indeed.

    *do* {} *while* (0) just couldn't return any value, it's not just a 
compound statement which gcc indeed allows to be evaluated.

> Sending v2 soon.

MBR, Sergei

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