linux-kernel - Re: [PATCH 5/8] staging: rtl8188eu: Remove function rtw

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Date:   Tue, 14 Apr 2020 15:22:59 +0300
From:   Ivan Safonov <insafonov@...il.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Nishka Dasgupta <nishkadg.linux@...il.com>,
        Larry Finger <Larry.Finger@...inger.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Michael Straube <straube.linux@...il.com>,
        devel@...verdev.osuosl.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/8] staging: rtl8188eu: Remove function rtw_modular64()

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.