linux-kernel - Re: [PATCH] staging: vt6656: Use BIT_ULL() macro instead of bit shift operation

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Date:   Sun, 8 Mar 2020 19:22:07 +0000
From:   Malcolm Priestley <tvboxspy@...il.com>
To:     Oscar Carter <oscar.carter@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Forest Bond <forest@...ttletooquiet.net>,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
        Gabriela Bittencourt <gabrielabittencourt00@...il.com>,
        Colin Ian King <colin.king@...onical.com>
Subject: Re: [PATCH] staging: vt6656: Use BIT_ULL() macro instead of bit shift
 operation

>>>   */
>>>  #undef __NO_VERSION__
>>>
>>> +#include <linux/bits.h>
>>>  #include <linux/etherdevice.h>
>>>  #include <linux/file.h>
>>>  #include "device.h"
>>> @@ -802,8 +803,7 @@ static u64 vnt_prepare_multicast(struct ieee80211_hw *hw,
>>>
>>>  	netdev_hw_addr_list_for_each(ha, mc_list) {
>>>  		bit_nr = ether_crc(ETH_ALEN, ha->addr) >> 26;
>>> -
>>> -		mc_filter |= 1ULL << (bit_nr & 0x3f);
>>> +		mc_filter |= BIT_ULL(bit_nr);
>>
>> Are you sure this does the same thing?  You are not masking off bit_nr
>> anymore, why not?
> 
> My reasons are exposed below:
> 
> The ether_crc function returns an u32 type (unsigned of 32 bits). Then the right
> shift operand discards the 26 lsb bits (the bits shifted off the right side are
> discarded). The 6 msb bits of the u32 returned by the ether_crc function are
> positioned in bit 5 to bit 0 of the variable bit_nr. Due to the right shift
> happens over an unsigned type, the 26 new bits added on the left side will be 0.
> 
> In summary, after the right bit shift operation we obtain in the variable bit_nr
> (unsigned of 32 bits) the value represented by the 6 msb bits of the value
> returned by the ether_crc function. So, only the 6 lsb bits of the variable
> bit_nr are important. The 26 msb bits of this variable are 0.
> 
> In this situation, the "and" operation with the mask 0x3f (mask of 6 lsb bits)
> is unnecessary due to its purpose is to reset (set to 0 value) the 26 msb bits
> that are yet 0.

The mask is only there out of legacy originally it was 31(0x1f) and the
bit_nr spread across two mc_filter u32 arrays.

The mask is not needed now it is u64.

The patch is fine.

Regards

Malcolm