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

FUJITA Tomonori
2009-06-01T08:08:25Z

On Mon, 1 Jun 2009 08:51:14 +0100
Russell King <rmk+lkml@arm.linux.org.uk> wrote:

> On Mon, Jun 01, 2009 at 01:02:42PM +0900, FUJITA Tomonori wrote:
> > Where can I find dma_coherent_dev?
> >
> > I don't fancy this since this is architecture-specific stuff (not
> > generic things).
>
> It _is_ very architecture specific. The coherent-ness of devices hardly
> depends on the device itself. Eg, PCI devices on x86 are coherent, but
> on ARM they aren't.
>
> > > +static inline void
> > > +dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
> > > + enum dma_data_direction direction)
> > > +{
> > > + debug_dma_unmap_page(dev, dma_addr, size, direction, true);
>
> Future ARMs will need to do something on unmaps.
>
> > > +static inline void
> > > +dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size,
> > > + enum dma_data_direction direction)
> > > +{
> > > + debug_dma_sync_single_for_cpu(dev, dma_handle, size, direction);
> > > +}
> > > +
> > > +static inline void
> > > +dma_sync_single_range_for_cpu(struct device *dev, dma_addr_t dma_handle,
> > > + unsigned long offset, size_t size,
> > > + enum dma_data_direction direction)
> > > +{
> > > + debug_dma_sync_single_range_for_cpu(dev, dma_handle,
> > > + offset, size, direction);
> > > +}
> >
> > This looks wrong. You put dma_coherent_dev hook in sync_*_for_device
> > but why you don't need it sync_*_for_cpu. It's architecture
> > specific. Some need both, some need either, and some need nothing.
>
> If you're non-coherent, you need to implement the sync APIs.
>
> > > +static inline int
> > > +dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
> > > +{
> > > + return 0;
>
> So mappings never fail?

Hmm, possibly, you might misunderstand what he is trying to do.

http://lkml.org/lkml/2009/5/28/393:

This adds a version of the dma-mapping API to asm-generic that can be
used by most architectures that only need a linear mapping.

=
The original thread is here (I'm not sure if we can have something
clean and useful like this header file):

http://lkml.org/lkml/2009/5/19/262

I think that ARM can't be an user of this (ARM needs more complicated
dma mapping stuff).