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SubjectRe: SLAB vs. pci_alloc_xxx in usb-uhci patch [RFC: API]


On Fri, 9 Mar 2001, David Brownell wrote:

> > > > > extern void *
> > > > > pci_pool_dma_to_cpu (struct pci_pool *pool, dma_addr_t handle);
> > > >
> > > > Do lots of drivers need the reverse mapping? It wasn't on my todo list
> > > > yet.
> > >
> > > Some hardware (like OHCI) talks to drivers using those dma handles.
> >
> > I wonder if it may be feasible to allocate a bunch of contiguous
> > pages. Then, whenever the hardware returns a bus address, subtract
> > the remembered bus address of the zone start, add the offset to
> > the virtual and voila.
>
> That's effectively what the implementation I posted is doing.
>
> Simple math ... as soon as you get the right "logical page",
> and that page size could become a per-pool tunable. Currently
> one logical page is PAGE_SIZE; there are some issues to
> deal with in terms of not crossing page boundaries.
>
> There can be multiple such pages, known to the pool allocator
> and hidden from the device drivers. I'd expect most USB host
> controllers wouldn't allocate more than one or two pages, so
> the cost of this function would typically be small.

Just for information to people that want to complexify the
pci_alloc_consistent() interface thats looks simple and elegant to me:
(Hopefully, I am not off topic here)

1) The sym53c8xx driver and friends expect this simple interface to
return naturally aligned memory chunks. It mostly allocates 1 page
at a time.

2) The sym* drivers use a very simple allocator that keeps track of bus
addresses for each chunk (page sized).
The object file of the allocator as seen in sym2 is as tiny as 3.4K
unstripped and 2.5K stripped.

3) The drivers need reverse virtual addresses for the DSA bus addresses
and implements a simplistic hashed list that costs no more
than 10 lines of trivial C code.

Btw, as a result, if pci_alloc_consistent() will ever return not
naturally aligned addresses the sym* drivers will be broken for
Linux.

This leaves me very surprised by all this noise given the _no_ issue I
have seen using the pci_alloc_consistent() interface. It looked to me a
_lot_ more simple to use than the equivalent interfaces I have had to
suffer with other O/Ses.

Now, if modern programmers are expecting Java-like interfaces for writing
kernel software, it is indeed another story. :-)

Gérard.

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