linux-kernel - Re: \\ 答复: [PATCH] of: del redundant type conversion

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Date:   Wed, 10 Apr 2019 22:30:21 -0700
From:   Frank Rowand <frowand.list@...il.com>
To:     xiaojiangfeng <xiaojiangfeng@...wei.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "robh@...nel.org" <robh@...nel.org>
Cc:     "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: \\ 答复: [PATCH] of: del redundant type conversion

On 4/10/19 9:21 PM, Frank Rowand wrote:
> On 4/10/19 9:13 PM, Frank Rowand wrote:
>> On 4/10/19 6:51 PM, xiaojiangfeng wrote:
>>> My pleasure.
>>>
>>> I am very new to sparse.
>>>
>>> I guess the warning is caused by the macro min.
>>
>> I think the warning is likely because the type of data is 'void *'.
>>
>> Removing the (int) cast is a good fix, but does not resolve
>> the sparse warning.
> 
> Let me correct myself.  When I ran sparse, I see the removing min() does
> eliminate the sparse warning.  I'm not sure why, so I'll go dig a little
> deeper.

Digging leaves me with more information, but still not sure of the actual
underlying cause.  min() is defined in include/linux/kernel.h.  Unraveling
the defines, the code that sparse is complaining about is in
__no_side_effects(), which is:

#define __no_side_effects(x, y) \
                (__is_constexpr(x) && __is_constexpr(y))

and __is_constexpr() is:

#define __is_constexpr(x) \
        (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))

The compiler warning points to the second sizeof() in the __is_constexpr() for
'l', which expands as:

  (sizeof(int) == sizeof(*(8 ? ((void *)((long)(   l) * 0l)) : (int *)8)))

I'll dig into this a little more, to see if maybe the problem is related to
my compiler version or sparse version.  Or if the reason lies elsewhere.

-Frank


> 
> -Frank
> 
>>
>> -Frank
>>
>>
>>> Then I submitted my changes.
>>>
>>> Thanks for code review.
>>>
>>>
>>> -----邮件原件-----
>>> 发件人: Frank Rowand [mailto:frowand.list@...il.com] 
>>> 发送时间: 2019年4月11日 2:50
>>> 收件人: xiaojiangfeng <xiaojiangfeng@...wei.com>; robh+dt@...nel.org; robh@...nel.org
>>> 抄送: devicetree@...r.kernel.org; linux-kernel@...r.kernel.org
>>> 主题: Re: [PATCH] of: del redundant type conversion
>>>
>>> On 4/10/19 1:29 AM, xiaojiangfeng wrote:
>>>> The type of variable l in early_init_dt_scan_chosen is int, there is 
>>>> no need to convert to int.
>>>>
>>>> Signed-off-by: xiaojiangfeng <xiaojiangfeng@...wei.com>
>>>> ---
>>>>  drivers/of/fdt.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c index 
>>>> 4734223..de893c9 100644
>>>> --- a/drivers/of/fdt.c
>>>> +++ b/drivers/of/fdt.c
>>>> @@ -1091,7 +1091,7 @@ int __init early_init_dt_scan_chosen(unsigned long node, const char *uname,
>>>>  	/* Retrieve command line */
>>>>  	p = of_get_flat_dt_prop(node, "bootargs", &l);
>>>>  	if (p != NULL && l > 0)
>>>> -		strlcpy(data, p, min((int)l, COMMAND_LINE_SIZE));
>>>> +		strlcpy(data, p, min(l, COMMAND_LINE_SIZE));
>>>>  
>>>>  	/*
>>>>  	 * CONFIG_CMDLINE is meant to be a default in case nothing else
>>>>
>>>
>>> Thanks for catching the redundant cast.
>>>
>>> There is a second problem detected by sparse on that line:
>>>
>>>   drivers/of/fdt.c:1094:34: warning: expression using sizeof(void)
>>>
>>> Can you please fix both issues?
>>>
>>> Thanks,
>>>
>>> Frank
>>>
>>
>>
> 
>