linux-kernel - Re: [PATCH v2 2/2] wifi: rtlwifi: rtl8821ae: phy: fix an undefined bitwise shift behavior

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Date:   Mon, 27 Nov 2023 09:13:09 +0800
From:   Su Hui <suhui@...china.com>
To:     Ping-Ke Shih <pkshih@...ltek.com>,
        "kvalo@...nel.org" <kvalo@...nel.org>,
        "nathan@...nel.org" <nathan@...nel.org>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        "dan.carpenter@...aro.org" <dan.carpenter@...aro.org>,
        "trix@...hat.com" <trix@...hat.com>
Cc:     "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>,
        "Larry.Finger@...inger.net" <Larry.Finger@...inger.net>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        "linville@...driver.com" <linville@...driver.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "lizetao1@...wei.com" <lizetao1@...wei.com>
Subject: Re: [PATCH v2 2/2] wifi: rtlwifi: rtl8821ae: phy: fix an undefined
 bitwise shift behavior

On 2023/11/24 19:19, Ping-Ke Shih wrote:
> On Fri, 2023-11-24 at 18:06 +0800, Su Hui wrote:
>> On 2023/11/24 16:51, Ping-Ke Shih wrote:
>>> Subject: [PATCH v2 2/2] wifi: rtlwifi: rtl8821ae: phy: fix an undefined bitwise shift behavior
>>>
>>> [...]
>>>> diff --git a/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c
>>>> b/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c
>>>> index 6df270e29e66..52ab1b0761c0 100644
>>>> --- a/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c
>>>> +++ b/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c
>>>> @@ -31,7 +31,12 @@ static u32 _rtl8821ae_phy_calculate_bit_shift(u32 bitmask)
>>>>    {
>>>>           u32 i = ffs(bitmask);
>>>>
>>>> -       return i ? i - 1 : 32;
>>>> +       if (!i) {
>>>> +               WARN_ON_ONCE(1);
>>>> +               return 0;
>>>> +       }
>>>> +
>>>> +       return i - 1;
>>>>    }
>>> Personally, I prefer to use __ffs(), because in normal case no need additional '-1',
>>> and abnormal cases should not happen.
>> Hi,  Ping-Ke
>>
>> Replace _rtl8821ae_phy_calculate_bit_shift() by __ffs(bitmask) is better,
>> but I'm not sure what callers should do when callers check bitmask is 0 before calling.
>> Maybe this check is useless?
>>
>> I can send a v3 patch if using  __ffs(bitmask) and no check for bitmask is fine.
>> Or could you send this patch if you have a better idea?
>> Thanks for your suggestion!
>>
> Can this work to you?
Looks good to me, briefer and better!
I will send v3 soon.
> static u32 _rtl8821ae_phy_calculate_bit_shift(u32 bitmask)
> {
> 	if (WARN_ON_ONCE(!bitmask))
> 		return 0;
>
> 	return __ffs(bitmask);
> }