Re: [RFC] generic device DMA implementation

[Adam J. Richter]

On Fri, 06 Dec 2002, James Bottomley wrote:
>[e-mail] said:
>> 	I like your term DMA_CONSISTENT better than DMA_CONFORMANCE_CONSISTANT
>> .  I think the word "conformance" in there does not reduce the time
>> that it takes to figure out what the symbol means.  I don't think any
>> other facility will want to use the terms DMA_{,IN}CONSISTENT, so I
>> prefer that we go with the more medium sized symbol. 

>I'm not so keen on this.  The idea of this parameter is not to tell the 
>allocation routine what type of memory you would like, but to tell it what 
>type of memory the driver can cope with.  I think for the inconsistent case, 
>DMA_INCONSISTENT looks like the driver is requiring inconsistent memory, and 
>expecting to get it.  I'm open to changing the "CONFORMANCE" part, but I'd 
>like to name these parameters something that doesn't imply they're requesting 
>a type of memory.

	How about renaming DMA_INCONSISTENT to DMA_MAYBE_CONSISTENT?

	By the way, I previously suggested a flags field to indicate
what the driver could cope with.  0 would mean consistent memory, 1's
would indicate other things that the driver could cope with that would
be added if and when a real need for them arises (read caching, write
back cachine, cpu-cpu consistency, cache line size smaller than 2**n
bytes, etc.).  Regarding the debugging capability of
DMA_CONFORMANCE_NONE, I don't think that will be as useful in the way
that DMA_DIRECTION_NONE is, because transfer direction is often passed
through the io path of a device driver and errors in doing so are a
common.  In comparison, I think the calls to dma_malloc will typically
have this argument specified as a constant where the call is made,
with the possible exception of some allocation being consolidated in
the generic device layer.

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