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

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Date:	Thu, 10 Dec 2015 20:26:19 +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 05:56 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?

    Anyway, this talk is quite pointless since the macro didn't return any 
value anyway.

MBR, Sergei

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