On Mon, 3 Jun 2019 14:47:06 +0200
Halil Pasic [off-list ref] wrote:

On Mon, 3 Jun 2019 13:37:45 +0200
Cornelia Huck [off-list ref] wrote:

quoted

On Wed, 29 May 2019 14:26:51 +0200
Michael Mueller [off-list ref] wrote:

quoted

quoted

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?

quoted

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.