[dpdk-dev] [PATCH] net: introduce big and little endian types

[Wiles, Keith]

> On Dec 6, 2016, at 8:45 AM, Ananyev, Konstantin <konstantin.ananyev at intel.com> wrote:
> 
> 
> 
>> 
>> On Tue, Dec 06, 2016 at 12:41:00PM +0000, Ananyev, Konstantin wrote:
>>> 
>>> 
>>>> -----Original Message-----
>>>> From: Richardson, Bruce
>>>> Sent: Tuesday, December 6, 2016 11:55 AM
>>>> To: Ananyev, Konstantin <konstantin.ananyev at intel.com>
>>>> Cc: Nélio Laranjeiro <nelio.laranjeiro at 6wind.com>; dev at dpdk.org; Olivier Matz <olivier.matz at 6wind.com>; Lu, Wenzhuo
>>>> <wenzhuo.lu at intel.com>; Adrien Mazarguil <adrien.mazarguil at 6wind.com>
>>>> Subject: Re: [dpdk-dev] [PATCH] net: introduce big and little endian types
>>>> 
>>>> On Tue, Dec 06, 2016 at 11:23:42AM +0000, Ananyev, Konstantin wrote:
>>>>> Hi Neilo,
>>>>> 
>>>>> 
>>>>> Hi Neilo,
>>>>>>>> 
>>>>>>>> This commit introduces new rte_{le,be}{16,32,64}_t types and updates
>>>>>>>> rte_{le,be,cpu}_to_{le,be,cpu}_*() and network header structures
>>>>>>>> accordingly.
>>>>>>>> 
>>>>>>>> Specific big/little endian types avoid uncertainty and conversion mistakes.
>>>>>>>> 
>>>>>>>> No ABI change since these are simply typedefs to the original types.
>>>>>>> 
>>>>>>> It seems like quite a lot of changes...
>>>>>>> Could you probably explain what will be the benefit in return?
>>>>>>> Konstantin
>>>>>> 
>>>>>> Hi Konstantin,
>>>>>> 
>>>>>> The benefit is to provide documented byte ordering for data types
>>>>>> software is manipulating to determine when network to CPU (or CPU to
>>>>>> network) conversion must be performed.
>>>>> 
>>>>> Ok, but is it really worth it?
>>>>> User can still make a mistake and forget to call ntoh()/hton() at some particular place.
>>>>> From other side most people do know that network protocols headers are usually in BE format.
>>>>> I would understand the effort, if we'll have some sort of tool that would do some sort of static code analysis
>>>>> based on these special types or so.
>>>>> Again, does it mean that we should go and change uint32_t to rte_le_32 inside all Intel PMDs
>>>>> (and might be  in some others too) to be consistent?
>>>>> Konstantin
>>>>> 
>>>> 
>>>> I actually quite like this patch as I think it will help make things
>>>> clear when the user is possibly doing something wrong. I don't think we
>>>> need to globally change all PMDs to use the types, though.
>>> 
>>> Ok, so where do you believe we should draw a line?
>>> Why let say inside lib/librte_net people should use these typedefs, but
>>> inside drivers/net/ixgbe they don't?
>> 
>> Because those are not public APIs. It would be great if driver writers
>> used the typedefs, but I don't think it should be mandatory.
> 
> Ok, so only public API would have to use these typedefs when appropriate, correct?
> I still think that the effort to make these changes and keep this rule outweighs the benefit,
> but ok if everyone else think it is useful - I wouldn't object too much. 

I believe the effort and advantages to this change have no real benefit when you can document the type in the function header. Adding a structure around the simple type just adds more typing and still will be difficult to manage even if it gives some compiler checking. The change would not prevent someone putting a BE value into a LE variable, right?

I would not like to see this type of change when documentation would be enough here. Breaking the ABI is a big thing here for a large number of APIs. We keep breaking the ABI and we need to stop doing it on every release of DPDK.

> 
>> 
>>> 
>>>> 
>>>> One thing I'm wondering though, is if we might want to take this
>>>> further. For little endian environments, we could define the big endian
>>>> types as structs using typedefs, and similarly the le types on be
>>>> platforms, so that assigning from the non-native type to the native one
>>>> without a transformation function would cause a compiler error.
>>> 
>>> Not sure I understand you here.
>>> Could you possibly provide some example?
>>> 
>> typedef struct {
>> 	short val;
>> } rte_be16_t;
> 
> Hmm, so:
> uint32_t x = rte_be_to_cpu_32(1);
> would suddenly stop compiling?
> That definitely looks like an ABI breakage to me.
> Konstantin
> 
>> 
>> That way if you try to assign a value of type rte_be16_t to a uint16_t
>> variable you'll get a compiler error, unless you use an appropriate
>> conversion function. In short, it changes things from not just looking
>> wrong - which is the main purpose of Neilo's patchset - to actually
>> making it incorrect from the compiler's point of view too.
>> 
>> /Bruce

Regards,
Keith