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SubjectRe: [PATCH] asm-generic: add a dma-mapping.h file
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On Mon, 18 May 2009 18:54:49 -0400
Jeff Garzik <jgarzik@pobox.com> wrote:

> Arnd Bergmann wrote:
> > h8300 and m32r currently do not provide a DMA mapping API
> > and therefore cannot use the ATA drivers. This adds a
> > generic version of dma-mapping.h for architectures that
> > have none or very minimal actual support for DMA in hardware
> > and makes the two architectures use it.
> >
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > ---
> > On Sunday 17 May 2009 20:05:54 Jeff Garzik wrote:
> >
> >> That's what needs to happen. We provide no-op functions for e.g. PCI
> >> and x86 DMI, for platforms where this support does not exist.
> >>
> >> Pretty much all architectures support some form of ATA. m68k, m32r,
> >> h8300 and microblaze all have IDE interface, which means that libata
> >> needs to work on that platform.
> >>
> >> The only !ATA arch in the entire kernel is s390, AFAICT.
> >
> > m68k only defines NO_DMA for Sun3 and Dragonball. Sun3 does
> > not have ATA, Dragonball could probably just enable HAS_DMA.
> >
> > ---
> > arch/h8300/Kconfig | 2 +-
> > arch/h8300/include/asm/dma-mapping.h | 1 +
> > arch/m32r/Kconfig | 2 +-
> > arch/m32r/include/asm/dma-mapping.h | 1 +
> > include/asm-generic/dma-mapping.h | 399 ++++++++++++++++++++++++++++++++++
> > 5 files changed, 403 insertions(+), 2 deletions(-)
> > create mode 100644 arch/h8300/include/asm/dma-mapping.h
> > create mode 100644 arch/m32r/include/asm/dma-mapping.h
> > create mode 100644 include/asm-generic/dma-mapping.h
>
> My main comment is a bit non-specific... I tend to think that all
> no-dma platforms should provide an API whose implementation always
> returns errors, e.g. an inlined version of dma-mapping-broken.h.
>
> That sort of setup permits the compiler's dead code elimination to work
> on these no-dma platforms, while not crapping up the libata code with a
> bunch of ifdefs.

Just adding one ifdef to libata.h works? What libata needs is just
wrapping dma_map_sg and dma_unmap_sg like the old ide stack does.

I'm not against the idea of an inlined version of dma-mapping-broken.h
but having two ways to handle this issue is a bit confusing; so far
what we done to handle this problem is using NO_DMA, like SCSI-ml
does, not putting the dma code in non-architecture code and having
something like scsi_lib_dma.c.

Is it theoretically correct that the dma mapping API lives in low
level drivers instead of libata-core (like how SCSI-ml does)?


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