Wrapper API for static bus_dma allocations
John Baldwin
jhb at freebsd.org
Fri Jan 30 20:31:45 UTC 2015
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.)
--
John Baldwin
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