Date: Fri, 30 Jan 2015 15:31:23 -0500 From: John Baldwin <jhb@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Poul-Henning Kamp <phk@phk.freebsd.dk>, 'freebsd-arch' <freebsd-arch@freebsd.org> Subject: Re: Wrapper API for static bus_dma allocations Message-ID: <1440008.gcoNUU8dV6@ralph.baldwin.cx> In-Reply-To: <20150130152150.GX42409@kib.kiev.ua> References: <2800970.jY4xzTy9Hz@ralph.baldwin.cx> <54CB9B9F.50905@FreeBSD.org> <20150130152150.GX42409@kib.kiev.ua>
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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@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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