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

[Ananyev, Konstantin]

> 
> 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. 

> 
> >
> > >
> > > 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