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From: Bruce Richardson <bruce.richardson@intel.com>
To: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>
Cc: =?iso-8859-1?Q?N=E9lio?= Laranjeiro <nelio.laranjeiro@6wind.com>,
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Adrien Mazarguil <adrien.mazarguil@6wind.com>
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Subject: Re: [dpdk-dev] [PATCH] net: introduce big and little endian types
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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@intel.com>
> > Cc: Nélio Laranjeiro <nelio.laranjeiro@6wind.com>; dev@dpdk.org; Olivier Matz <olivier.matz@6wind.com>; Lu, Wenzhuo
> > <wenzhuo.lu@intel.com>; Adrien Mazarguil <adrien.mazarguil@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.
>
> >
> > 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;
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