Re: [RFC] generic device DMA implementation

[David Gibson]

On Wed, Dec 04, 2002 at 11:47:14AM -0600, James Bottomley wrote:
> Currently our only DMA API is highly PCI specific (making any non-pci bus with 
> a DMA controller create fake PCI devices to help it function).
> 
> Now that we have the generic device model, it should be equally possible to 
> rephrase the entire API for generic devices instead of pci_devs.
> 
> This patch does just that (for x86---although I also have working code for 
> parisc, that's where I actually tested the DMA capability).
> 
> The API is substantially the same as the PCI DMA one, with one important 
> exception with regard to consistent memory:
> 
> The PCI api has pci_alloc_consistent which allocates only consistent memory 
> and fails the allocation if none is available thus leading to driver writers 
> who might need to function with inconsistent memory to detect this and employ 
> a fallback strategy.
> 
> The new DMA API allows a driver to advertise its level of consistent memory 
> compliance to dma_alloc_consistent.  There are essentially two levels:
> 
> - I only work with consistent memory, fail if I cannot get it, or
> - I can work with inconsistent memory, try consistent first but return 
> inconsistent if it's not available.

Do you have an example of where the second option is useful?  Off hand
the only places I can think of where you'd use a consistent_alloc()
rather than map_single() and friends is in cases where the hardware's
behaviour means you absolutely positively have to have consistent
memory.

-- 
David Gibson			| For every complex problem there is a
[e-mail]	| solution which is simple, neat and
				| wrong.
http://www.ozlabs.org/people/dgibson