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

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Message-Id: <20090527125801B.fujita.tomonori@lab.ntt.co.jp>
Date:	Wed, 27 May 2009 12:58:46 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	arnd@...db.de
Cc:	fujita.tomonori@....ntt.co.jp, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] asm-generic: add a dma-mapping.h file

On Tue, 26 May 2009 13:35:41 +0100
Arnd Bergmann <arnd@...db.de> wrote:

> 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

Seems that people think regard #ifdef in *.c bad, I think.


> 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

This example is inappropriate; a different way to use #ifdef.

This just ensures that dma_debug_add_bus() is available even if
CONFIG_DMA_API_DEBUG is disabled.

It's completely different from a way how you use #ifdef.


> #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

#define clear_user_highpage(page,vaddr)		\
	 __cpu_clear_user_highpage(page, vaddr)

Personally, I don't fancy this style but it's a matter of taste, I
guess.


> 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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