linux-kernel - Re: [PATCH 2/3] net:ethernet:rmnet:Support for downlink MAPv5 csum offload

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Date:   Fri, 12 Feb 2021 10:49:49 -0700
From:   subashab@...eaurora.org
To:     Alex Elder <elder@...e.org>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Sharath Chandra Vurukala <sharathv@...eaurora.org>,
        davem@...emloft.net, elder@...nel.org, cpratapa@...eaurora.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] net:ethernet:rmnet:Support for downlink MAPv5 csum
 offload

On 2021-02-12 07:01, Alex Elder wrote:
> On 2/11/21 8:04 PM, Jakub Kicinski wrote:
>> On Fri, 12 Feb 2021 03:05:23 +0530 Sharath Chandra Vurukala wrote:
>>> +/* MAP CSUM headers */
>>> +struct rmnet_map_v5_csum_header {
>>> +	u8  next_hdr:1;
>>> +	u8  header_type:7;
>>> +	u8  hw_reserved:5;
>>> +	u8  priority:1;
>>> +	u8  hw_reserved_bit:1;
>>> +	u8  csum_valid_required:1;
>>> +	__be16 reserved;
>>> +} __aligned(1);
>> 
>> Will this work on big endian?
> 
> Sort of related to this point...
> 
> I'm sure the response to this will be to add two versions
> of the definition, surrounded __LITTLE_ENDIAN_BITFIELD
> and __BIG_ENDIAN_BITFIELD tests.
> 
> I really find this non-intuitive, and every time I
> look at it I have to think about it a bit to figure
> out where the bits actually lie in the word.
> 
> I know this pattern is used elsewhere in the networking
> code, but that doesn't make it any easier for me to
> understand...
> 
> Can we used mask, defined in host byte order, to
> specify the positions of these fields?
> 
> I proposed a change at one time that did this and
> this *_ENDIAN_BITFIELD thing was used instead.
> 
> I will gladly implement this change (completely
> separate from what's being done here), but thought
> it might be best to see what people think about it
> before doing that work.
> 
> 					-Alex

Our preference is to stick with __LITTLE_ENDIAN_BITFIELD
& __BIG_ENDIAN_BITFIELD definitions similar to other
networking definitions.