linux-kernel - Re: [PATCH] asm-generic: add a dma-mapping.h file

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Message-Id: <200905261335.41643.arnd@arndb.de>
Date:	Tue, 26 May 2009 13:35:41 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] asm-generic: add a dma-mapping.h file

On Tuesday 26 May 2009, FUJITA Tomonori wrote:
> > dma_get_cache_alignment() is still less generic than
> > the other functions, as this is still architecture
> > specific. Should I leave that out as well then?
> 
> Yes, I think that only adding generic functions is a better
> approach. Overriding with #ifdef is really ugly.
> 

Ok, will do.

[trimming Cc list a bit, as the subject has changed]

Could you be more specific as to why you think the #ifdef
in this file is ugly? I agree that we should always avoid
#ifdef in a device driver file or around the usage of an
API, but we use it everywhere when defining an optional
API, e.g.

#ifdef CONFIG_DMA_API_DEBUG
extern void dma_debug_add_bus(struct bus_type *bus);
#else
static inline void dma_debug_add_bus(struct bus_type *bus)
{
{
#endif

or

#ifndef clear_user_highpage
static inline void clear_user_highpage(struct page *page, unsigned long vaddr)
{
	void *addr = kmap_atomic(page, KM_USER0);
	clear_user_page(addr, vaddr, page);
	kunmap_atomic(addr, KM_USER0);
}
#endif

This has always made a lot of sense to me when you are defining
something that you want to become a No-op in certain configurations
or that only needs special handling in a few cases. IIRC, Linus
has stated that he prefers the second of the two styles above when
you have the choice, which is why I started out that way, while
for the dma_debug stuff, the CONFIG_ symbol clearly makes sense.

	Arnd <><
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