Re: [PATCH v3 2/8] s390/cio: introduce DMA pools to cio

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 3 Jun 2019 14:47:06 +0200
Halil Pasic <pasic@linux.ibm.com> wrote:

> On Mon, 3 Jun 2019 13:37:45 +0200
> Cornelia Huck <cohuck@redhat.com> wrote:
> 
> > On Wed, 29 May 2019 14:26:51 +0200
> > Michael Mueller <mimu@linux.ibm.com> wrote:

> > > diff --git a/arch/s390/include/asm/cio.h b/arch/s390/include/asm/cio.h
> > > index 1727180e8ca1..43c007d2775a 100644
> > > --- a/arch/s390/include/asm/cio.h
> > > +++ b/arch/s390/include/asm/cio.h
> > > @@ -328,6 +328,17 @@ static inline u8 pathmask_to_pos(u8 mask)
> > >  void channel_subsystem_reinit(void);
> > >  extern void css_schedule_reprobe(void);
> > >  
> > > +extern void *cio_dma_zalloc(size_t size);
> > > +extern void cio_dma_free(void *cpu_addr, size_t size);
> > > +extern struct device *cio_get_dma_css_dev(void);
> > > +
> > > +struct gen_pool;  
> > 
> > That forward declaration is a bit ugly...   
> 
> Can you explain to me what is ugly about it so I can avoid similar
> mistakes in the future?
> 
> >I guess the alternative was
> > include hell?
> >   
> 
> What do you mean by include hell?
> 
> I decided to use a forward declaration because the guys that include
> "cio.h" are not expected to require the interfaces defined in
> linux/genalloc.h. My motivation to do it like this was the principle of
> encapsulation.

My personal rule-of-thumb is to include the header if it is
straightforward enough (e.g. if adding a basic header is enough). If
you need to include a header together with all of its friends and
family, a forward declaration is probably nicer. And of course,
sometimes it is simply needed.