netdev - Re: [ovs-dev] [PATCH net-next V2 1/1] openvswitch: Declare ovs key structures using macros

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Date:   Thu, 7 Feb 2019 05:47:05 +0000
From:   Eli Britstein <elibr@...lanox.com>
To:     Ben Pfaff <blp@....org>, Gregory Rose <gvrose8192@...il.com>
CC:     David Miller <davem@...emloft.net>,
        "yihung.wei@...il.com" <yihung.wei@...il.com>,
        "dev@...nvswitch.org" <dev@...nvswitch.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "simon.horman@...ronome.com" <simon.horman@...ronome.com>
Subject: Re: [ovs-dev] [PATCH net-next V2 1/1] openvswitch: Declare ovs key
 structures using macros


On 2/5/2019 10:23 PM, Ben Pfaff wrote:
> On Tue, Feb 05, 2019 at 10:22:09AM -0800, Gregory Rose wrote:
>> On 2/5/2019 4:02 AM, Eli Britstein wrote:
>>> On 2/4/2019 10:07 PM, David Miller wrote:
>>>> From: Yi-Hung Wei <yihung.wei@...il.com>
>>>> Date: Mon, 4 Feb 2019 10:47:18 -0800
>>>>
>>>>> For example, to see how 'struct ovs_key_ipv6' is defined, now we need
>>>>> to trace how OVS_KEY_IPV6_FIELDS is defined, and how OVS_KEY_FIELD_ARR
>>>>> and OVS_KEY_FIELD defined.  I think it makes the header file to be
>>>>> more complicated.
>>>> I completely agree.
>>>>
>>>> Unless this is totally unavoidable, I do not want to apply a patch
>>>> which makes reading and auditing the networking code more difficult.
>>> This technique is discussed for example in
>>> https://stackoverflow.com/questions/6635851/real-world-use-of-x-macros,
>>> and I found existing examples of using it in the kernel tree:
>>>
>>> __BPF_FUNC_MAPPER in commit ebb676daa1a34 ("bpf: Print function name in
>>> addition to function id")
>>>
>>> __AAL_STAT_ITEMS and __SONET_ITEMS in commit 607ca46e97a1b ("UAPI:
>>> (Scripted) Disintegrate include/linux"), the successor of commit
>>> 1da177e4c3f4 ("Linux-2.6.12-rc2"). I didn't dig deeper to the past.
>>>
>>> I can agree it makes that H file a bit more complicated, but for sure
>>> less than ## macros that are widely used.
>>>
>>> However, I think the alternatives of generating such defines by some
>>> scripts, or having the fields in more than one place are even worse, so
>>> it is a kind of unavoidable.
>> Why is using a script to generate defines for the requirements of your
>> application or driver so bad?  Your patch
>> turns openvswitch.h into some hardly readable code - using a script and
>> having a machine output the macros
>> your application or driver needs seems like a much better alternative to me.
OK, let's abandon this patch. I'll go with the script alternative.
> In addition, one of the reasons that developers prefer to avoid
> duplication is because of the need to synchronize copies when one of
> them changes.  This doesn't apply in the same way to these structures,
> because they are ABIs that will not change.
This is correct, but still, though it is not likely to change, it might 
will, so I think we must avoid duplication.