Re: [dpdk-dev] [PATCH v2] mbuf: decrease refcnt when detaching

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> Sent: Tuesday, May 17, 2016 3:19 PM
> To: Ananyev, Konstantin
> Cc: dev@dpdk.org; Hiroyuki Mikita; olivier.matz@6wind.com
> Subject: Re: [dpdk-dev] [PATCH v2] mbuf: decrease refcnt when detaching
> 
> 2016-05-17 13:44, Ananyev, Konstantin:
> > From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> > > 2016-05-17 12:59, Ananyev, Konstantin:
> > > > > > The rte_pktmbuf_detach() function should decrease refcnt on a direct
> > > > > > buffer.
> > > > >
> > > > > As you have noticed, "whenever the indirect buffer is detached,
> > > > > the reference counter on the direct buffer is decremented."
> > > > > So the current behaviour of rte_pktmbuf_detach() is buggy.
> > > > > Why not fix it without renaming?
> > > > > If you consider this behavioral bug is part of the API, we
> > > > > can fix it in a new function unattach and deprecate detach.
> > > > > But Konstantin, why do you want to keep a restore function?
> > > > > What is the need?
> > > >
> > > > I think it might be a useful functionality in some situations:
> > > > some users can attach/detach to external memory buffers (no mbufs)
> > > > and similar functionality is required.
> > >
> > > Attach to external memory buffer (raw buffer) is not currently supported.
> > >
> > > > Let say right now examples/vhost/main.c has its own pktmbuf_detach_zcp()
> > >
> > > You should look at the commit http://dpdk.org/commit/68363d85
> > > 	"examples/vhost: remove the non-working zero copy code"
> > >
> > > > which is doing pretty much the same - restore original values, after detaching
> > > > mbuf from external (virtio) memory buffer.
> > > > Would be good if we'll use a standard API function here.
> > >
> > > You are welcome to implement mbuf attach to raw buffer.
> > > But it is not a requirement for this fix.
> >
> > Hmm, still not sure why we can't keep an existing function?
> 
> Because it does not do what its name (and doc) suggest.
> 
> > Obviously it wouldn't cost anything and I still think might be useful.
> 
> It costs to overcomplicate API for only a half support.

I still think it is better to have it then not, but wouldn't insist here.
Konstantin

> If you need the feature "attach to raw", please implement it completely.