Re: [PATCH 2/3] dma: override "dma_flags_set_dmaflush" for sn-ia64

James Bottomley
2007-08-22T18:13:04Z

On Wed, 2007-08-22 at 10:17 -0700, Jesse Barnes wrote:
> On Wednesday, August 22, 2007 10:04:32 am James Bottomley wrote:
> > The spec isn't ambiguous ... it says if the device and bridge agree on
> > relaxed ordering, then it *may* be observed in the transaction. If
> > there's a disagreement or neither wishes to support relaxed ordering
> > then the transaction *must* be strict.
>
> Arg, don't make me dig out my spec, I don't have it handy...

Well ... I don't have one either. However, Grant Grundler did a
presentation to OLS about relaxed ordering, and I went over it pretty
thoroughly with him a while ago. The bottom line is that the default is
always strict unless both the bridge and the device agree otherwise.

> > I really don't think a work around for a PCI spec violation belongs in
> > the generic DMA code, do you? The correct fix for this should be to set
> > the device hints to strict ordering, which presumably altix respects?
>
> Well, the Altix hw by itself won't honor device hints (I'm not even sure if
> there are devices that honor order hints like you outline above). However,
> Altix could track per-device ordering as long as arch code was called from
> such a hook.
>
> > In which case, it sounds like what needs exposing are access to the PCI
> > device hints. I believe both PCI-X and PCIe have these hints as
> > optional specifiers in the bridges, so it should be in a current Rev of
> > the PCI spec. Or are you proposing adding an additional PCI API that
> > allows transaction flushes to be inserted into the stream for devices
> > and bridges that have already negotiated relaxed ordering? ... in which
> > case we need to take this to the PCI list.
>
> I think it would have to be the latter, since otherwise it would be hard to
> setup just completion queue requests to be ordered.

OK ... I think this is definitely a PCI specific API ... and probably a
generic one for requesting order flushes in devices that have negotiated
relaxed ordering. Do you want to start a new thread on linux-pci and cc
me?

James