returned dma address value of dma_alloc_coherent, is it always in the lower 4GB range?

["Chan Kim" <ckim@etri.re.kr>]

Hello experts,

I was reading the well known DMA-API-HOWTO document
(https://www.kernel.org/doc/Documentation/DMA-API-HOWTO.txt),
(I've read this document a couple of times before in the past.)

In the " Using Consistent DMA mappings" section it says :
-----------------------------
	dma_addr_t dma_handle;

	cpu_addr = dma_alloc_coherent(dev, size, &dma_handle, gfp);
...
The consistent DMA mapping interfaces, will by default return a DMA address
which is 32-bit addressable.  Even if the device indicates (via the DMA
mask)
that it may address the upper 32-bits, consistent allocation will only
return > 32-bit addresses for DMA if the consistent DMA mask has been
explicitly changed via dma_set_coherent_mask().  This is true of the
dma_pool interface as well.
------------------------------

I understand by 'DMA address', it means dma_handle (the address the device
will use for DMA).
I can't understand the expression "consistent allocation will only return >
32-bit addresses".
Does it mean that when the driver sets the coherent_dma_mask, for example,
to DMA_BIT_MAST(64) and it was successful, does it return the dma_handle in
lower 32 bit address? What does "return > 32-bit addresses" mean? The ">"
syntax here is so obscure with no clear definition of usage.
By context I guess it returns the address in lower 32bit address range even
it the coherent_dma_mask is set to some bigger value.
(And I know generally, dma address(= bus address) is not equal to the
physical address and is assigned by the OS for convenience)

Thank you.
Chan Kim




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