linux-kernel - Re: [PATCH] Staging: rtl8723au: os_intfs: fixed case statement is variable issue

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Date:	Sat, 13 Aug 2016 17:26:44 +0800
From:	sunbing <sunbing@...flag-linux.com>
To:	Jes Sorensen <Jes.Sorensen@...hat.com>
Cc:	Larry.Finger@...inger.net, gregkh@...uxfoundation.org,
	linux-wireless@...r.kernel.org, devel@...verdev.osuosl.org,
	linux-kernel@...r.kernel.org, sunbing.linux@...il.com
Subject: Re: [PATCH] Staging: rtl8723au: os_intfs: fixed case statement is variable issue


On Aug 12, 2016, at 22:30, Jes Sorensen <Jes.Sorensen@...hat.com> wrote:

> sunbing <sunbing@...flag-linux.com> writes:
>> On Aug 11, 2016, at 23:25, Jes Sorensen <Jes.Sorensen@...hat.com> wrote:
>> 
>>> Bing Sun <sunbing@...flag-linux.com> writes:
>>>> Fixed sparse parse error:
>>>> Expected constant expression in case statement.
>>>> 
>>>> Signed-off-by: Bing Sun <sunbing@...flag-linux.com>
>>>> ---
>>>> drivers/staging/rtl8723au/os_dep/os_intfs.c | 11 +++++------
>>>> 1 file changed, 5 insertions(+), 6 deletions(-)
>>>> 
>>>> diff --git a/drivers/staging/rtl8723au/os_dep/os_intfs.c b/drivers/staging/rtl8723au/os_dep/os_intfs.c
>>>> index b8848c2..f30d5d2 100644
>>>> --- a/drivers/staging/rtl8723au/os_dep/os_intfs.c
>>>> +++ b/drivers/staging/rtl8723au/os_dep/os_intfs.c
>>>> @@ -283,14 +283,13 @@ static u32 rtw_classify8021d(struct sk_buff *skb)
>>>> 	 */
>>>> 	if (skb->priority >= 256 && skb->priority <= 263)
>>>> 		return skb->priority - 256;
>>>> -	switch (skb->protocol) {
>>>> -	case htons(ETH_P_IP):
>>>> +
>>>> +	if (skb->protocol == htons(ETH_P_IP)) {
>>>> 		dscp = ip_hdr(skb)->tos & 0xfc;
>>>> -		break;
>>>> -	default:
>>>> -		return 0;
>>>> +		return dscp >> 5;
>>>> 	}
>>>> -	return dscp >> 5;
>>>> +
>>>> +	return 0;
>>>> }
>>> 
>>> Pardon me here, but I find it really hard to see how this change is an
>>> improvement over the old code in any shape or form.
>>> 
>>> Jes
>> 
>> There is no functional improvement. 
>> But before this patch, when we do: make C=1 M=drivers/staging/rtl8723au/
>> An error output: 
>> drivers/staging/rtl8723au//os_dep/os_intfs.c:287:14: error: Expected
>> constant expression in case statement
>> To avoid sparse parse error, a case statement converts to an if statement.
>> So we got this patch.
> 
> Hello
> 
> I understand this part, but it seems to me we are changing the code due
> to a broken test case in sparse. Does the warning go away if you use
> __constant_htons() instead of htons()?
> 
> Jes

Thanks for your guidance.

1. If I use __constant_htons, checkpatch.pl will warning:
    WARNING: __constant_htons should be htons

2. In os_intfs.c: rtw_classify8021d, there are only one case statement and a 
default statement. So, convert "switch case" to "if else" is more readable in my opinion.

So, I pushed this patch.

There are some patches convert use of __constant_htons to htons in kernel logs. 
Will there be a new patch convert to htons in the future if I use  __constant_htons now ?

After search through kernel code, there are 158 "case htons(...)" statements and 
2 "case __constant_htons(...)" statements. Does this mean we can ignore sparse 
error and use "case htons(...)" ?

It makes me confused. More help, please.

Regards.