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

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Date:	Mon, 8 Jun 2009 14:49:17 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	arnd@...db.de
Cc:	fujita.tomonori@....ntt.co.jp, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH] asm-generic: add dma-mapping-linear.h

On Thu, 4 Jun 2009 17:47:03 +0100
Arnd Bergmann <arnd@...db.de> wrote:

> On Thursday 04 June 2009, FUJITA Tomonori wrote:
> > On Thu, 4 Jun 2009 12:35:34 +0000
> > Arnd Bergmann <arnd@...db.de> wrote:
> > 
> > > > > I do have branches in there that convert x86 and microblaze to use
> > > > 
> > > > x86? How can it use asm-generic-linear.h?
> > > 
> > > Not dma-mapping-linear.h, but a lot of other files in asm-generic. I have
> > > a set of all the common header files in my tree, and x86 can e.g. use
> > > ioctls.h, ipcbuf.h, mman.h, or types.h. For x86, it amounts to 15 files,
> > > microblaze can use almost 50 of the generic files.
> > 
> > I see, but I'm not sure why dma-mapping-linear needs to be merged with
> > them together.
> 
> It doesn't need to, but it would be much more convenient for me having
> to go through the architectures only once. Some of the arch maintainers
> are harder to get hold of than others.
> 
> > > 
> > > I think it is technically correct, but there are two plausible ways of
> > > doing it, I chose the one that requires slightly less code.
> > > 
> > > I call dma_cache_sync() for streaming mappings on dma_map_* and
> > > dma_sync_*_for_device iff the mapping is noncoherent. AFAICT, this
> > > is the same case as dma_alloc_noncoherent, which is expected to give
> > > out a noncoherent mapping.
> > 
> > If I correctly understand DMA-API.txt, dma_alloc_noncoherent can
> > return either consistent or non-consistent memory. On architectures
> > that return consistent memory via dma_alloc_noncoherent,
> > dma_cache_sync should be null. dma_cache_sync() is supposed to be used
> > only with the returned buffers of dma_alloc_noncoherent().
> 
> Good point. This is unfortunately not what is implemented on many
> architectures, which #define dma_alloc_noncoherent dma_alloc_coherent
> but still provide a synchronizing operation in dma_cache_sync().
> 
> dma_alloc_noncoherent is actually only implemented on parisc, mips
> and m68k.
> 
> However, I don't think I have the energy to fix this problem, but
> I agree that it should be fixed eventually. I can leave out
> the declarations of dma_{free,alloc}_coherent from dma-mapping-linear.h
> so that the broken code remains in the architecture specific
> files, and change all references to dma_cache_sync to something
> else. The best I can think of is __dma_cache_sync() with the same
> calling conventions as dma_cache_sync(). Does that make sense?

Sorry, but it doesn't make sense to me because __dma_cache_sync() hack
is against the goal of dma-mapping-linear.h, having a clean, ideal,
unified header file.
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