On Fri, 2010-02-05 at 16:43 +0000, James Bottomley wrote:
> On Fri, 2010-02-05 at 16:31 +0000, Catalin Marinas wrote:
> > This patch introduces support for the PIO mapping API on the ARM
> > architecture. It is currently only meant as an example for discussions
> > and it can be further optimised.
[...]
> > diff --git a/arch/arm/include/asm/pio-mapping.h b/arch/arm/include/asm/pio-mapping.h
> > new file mode 100644
> > index 0000000..d7c866a
> > --- /dev/null
> > +++ b/arch/arm/include/asm/pio-mapping.h
[...]
> > +static inline void *pio_map_single(void *addr, size_t size,
> > + enum pio_data_direction dir)
> > +{
> > + return addr;
>
> This API is a bit semantically nasty to use, isn't it? What we usually
> get in the I/O path is a scatter gather list of pages and offsets (or
> just a page in the network case).
>
> In a highmem kernel, we'd have to kmap the page before it actually had a
> kernel virtual address, so now the use case of the API becomes
>
> vaddr = kmap_...(page)
> pio_map_single(vaddr)
>
> and the reverse on unmap.
>
> Why not just combine the two since we always have to do them anyway and
> do
>
> kmap_pio ... with the various atomic versions?
For most cases, what we need looks close enough to the kmap semantics
but this wouldn't work for the HCD case - the USB mass storage may use
kmap() but it cannot easily be converted to the kmap_pio API since it's
only the HCD driver that knows what kind of transfer it would do (and
this only gets a (void *) pointer).
We could allow a kmap_pio API to also take (void *) arguments in
addition to (struct page *) but would this be confusing? It diverges
slightly from the kmap API. Do you have a better suggestion?
> enum pio_data_direction dir looks a bit wrong too ... why not just use
> the exsiting enum dma_data_direction since that's what the dma API uses?
I just didn't want to confuse things with the DMA_* name but that's a
valid point.
> your pio_map_page is going to have to contain a kmap in the arch
> implementation anyway ...
In my approach, the only case for pio_map_page() would be for
non-highmem pages, otherwise you would need to use pio_map_single() on
the value returned by kmap() (as you pointed out, it doesn't look nice
but I don't have a solution to accommodate a kmap API with the HCD
drivers).
Thanks.
--
Catalin
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