Wrapper API for static bus_dma allocations

[Warner Losh]

> On Jan 30, 2015, at 2:31 PM, John Baldwin <jhb at freebsd.org> wrote:
> 
> On Friday, January 30, 2015 05:21:50 PM Konstantin Belousov wrote:
>> On Fri, Jan 30, 2015 at 09:56:31AM -0500, John Baldwin wrote:
>>> On 1/29/15 4:54 PM, Poul-Henning Kamp wrote:
>>>> --------
>>>> 
>>>> In message <2800970.jY4xzTy9Hz at ralph.baldwin.cx>, John Baldwin writes:
>>>>> The bus_dma API to allocate a chunk of static DMA'able memory (e.g. for
>>>>> descriptor rings) can be a bit obtuse [...]
>>>> 
>>>> Isn't it time we take a good hard stare at all of the bus_dma API,
>>>> and refactor it into something a lot more compact ?
>>> 
>>> Given the amount of oddball hardware out there I don't think there is a
>>> lot you can cut out.  The filter function might be something we can lose
>>> (and losing it would simplify the implementation), but all the other
>>> weird constraints are actually used by something AFAIK.  I do think we
>>> can provide some simpler wrappers for some of the more common cases, but
>>> there will be some hardware for which those wrappers do not work.
>>> 
>>> One suggestion Scott has had is to at least make it easier to extend the
>>> API by using getter/setter routines on the tag to work with tag
>>> attributes instead of passing them all in bus_dma_tag_create().
>> 
>> BTW, filter function is useless.  It can deny specific bus address from
>> being used, but it does not provide the busdma implementation even a hint
>> what other address should be (tried to) used.  In dmar busdma, I simply
>> ignored it.  And there is no real users of filter in the tree.
> 
> Yes, it is very annoying.  I think some old ISA SCSI HBA driver might have 
> used it to skip over some low-memory hole (i.e. there were two valid DMA 
> ranges and this was the kludge instead of having two sets of lowaddr/highaddr 
> exclusions).  (That is one part of the API we could rototill is to just remove 
> the highaddr arg just use a single arg which is effectively lowaddr.  I think 
> all drivers always set highaddr to BUS_SPACE_MAXADDR.)

Not all. There’s some PCI cards that can’t do 64-bit cycles that pass in the 32-bit
value on 64-bit systems. There’s 386 instances of this in the tree. But that may be
lowaddr only. It’s hard to grep for this to be sure.

Wanrer