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Subject: Re: [PATCH 5/8] staging: rtl8188eu: Remove function rtw_modular64()
To:     Dan Carpenter <dan.carpenter@oracle.com>
Cc:     Nishka Dasgupta <nishkadg.linux@gmail.com>,
        Larry Finger <Larry.Finger@lwfinger.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Michael Straube <straube.linux@gmail.com>,
        devel@driverdev.osuosl.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20190712071746.2474-5-nishkadg.linux@gmail.com>
 <66fb1e37-107e-54f4-4986-ca4e4bb99d2d@yandex.ru>
 <20200414115604.GF1163@kadam>
From:   Ivan Safonov <insafonov@gmail.com>
Message-ID: <20becb6f-7bf6-8618-9525-b64c226088c7@gmail.com>
Date:   Tue, 14 Apr 2020 15:22:59 +0300
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On 4/14/20 2:56 PM, Dan Carpenter wrote:
> On Sun, Apr 12, 2020 at 04:34:08PM +0300, Ivan Safonov wrote:
>>> Remove function rtw_modular64 as all it does is call do_div.
>>
>> This is wrong. Macro do_div(x, y) change first argument x, but
>> rtw_modular64(x, y) preserve it.
>>
>>> +			tsf = pmlmeext->TSFValue - do_div(pmlmeext->TSFValue, (pmlmeinfo->bcn_interval*1024)) - 1024; /* us */
>>
>> rounddown(pmlmeext->TSFValue, pmlmeinfo->bcn_interval * 1024) - 1024
>> is a better replacement for
> 
> You're absolutely correct that the patch is buggy, but I'm not sure that
> rounddown() is what we want.
> 
> rtw_modular64() took the MOD of x.  So it should be something like:
> 
> 	tsf = pmlmeext->TSFValue - (pmlmeext->TSFValue % (pmlmeinfo->bcn_interval * 1024)) - 1024; /* us */
> 
> But what the heck is that even???  If pmlmeinfo->bcn_interval is zero
> or one then the subtraction ends up giving us a negative.
> 
> regards,
> dan carpenter
> 

1. pmlmeext->TSFValue can not be negative, because it is uint64_t;
2. pmlmeext->TSFValue is cyclic value:
     https://en.wikipedia.org/wiki/Timing_synchronization_function ;
3. (rounddown(a, b)) is equal to (a - a % b) by definition.

Ivan Safonov.