Problems with dma_alloc_coherent()
jwhitney-linuxppc at sands-edge.com
Sat Apr 3 02:50:27 EST 2004
As an exemplification of the problem I am having with the current
I am developing a Generic DMA driver. DMA providers register
themselves with it. Clients ask the DMA core driver to please have a
registered driver transfer the data from one location to another, and
call them back when it is complete.
Now, let's look at my options:
1. The DMA-core API accepts physical addresses only for source and
This technically can't be done. The virt_to_phys() API, I am told, is
deprecated and shouldn't be used by the kernel. However, the client
code can't call dma_map_single() or dma_alloc_coherent(), because it
doesn't KNOW which providers are available and will be servicing the
2. Okay, we'll make the DMA-core API accept only virtual addresses,
which it can map itself to the final driver.
Okay... this works well for the local memory (__get_free_pages() and
all), but the client has a physical address of a device it wants to
write to... how does it get a virtual address? ioremap(). Bzzt! The
address returned by ioremap is fully virtual, which dma_map_single()
can't accept (as it calls virt_to_bus(), which doesn't handle
3. Well, darn. How about the client calls
dma_map_single/dma_alloc_coherent with a NULL device, to get a physical
address for allocated local memory to pass to the DMA core?
Well, that sort of works. However, do ALL platforms add the PCI bus
address-map-offset to the value? How does the core (processor
independent) code know what to subract off to get back to the base
physical address? There is no bus_to_phys() to phys_to_bus() routines,
and these would be deprecated even if they did exist. How does it know
what to add to get back a bus-offset-phys-address suitable for the DMA
controller? I guess the DMA core COULD call bus_to_virt() (although it
is deprecated), and then call dma_map_single() for the appropriate
4. I suppose I COULD have a far more complicated dma_mem_to_dev() and
dma_dev_to_mem() and dma_dev_to_dev() and dma_mem_to_mem() kind of
interface, which mixed virtual and physical addresses. Ugh.
Am I missing anything that should have made this all simpler? If the
DMA/PCI API routines simply accepted ANY virtual address (fully-virtual
or semi-virtual), I wouldn't have this problem. I'd simply accept
virtual addresses for the source and destination.
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