Re: [RFT PATCH v3 12/27] of/address: Add infrastructure to declare MMIO as non-posted

[Rob Herring]

On Wed, Mar 10, 2021 at 1:27 AM Hector Martin <marcan@xxxxxxxxx> wrote:
>
> On 10/03/2021 07.06, Rob Herring wrote:
> >> My main concern here is that this creates an inconsistency in the device
> >> tree representation that only works because PCI drivers happen not to
> >> use these code paths. Logically, having "nonposted-mmio" above the PCI
> >> controller would imply that it applies to that bus too. Sure, it doesn't
> >> matter for Linux since it is ignored, but this creates an implicit
> >> exception that PCI buses always use posted modes.
> >
> > We could be stricter that "nonposted-mmio" must be in the immediate
> > parent. That's kind of in line with how addressing already works.
> > Every level has to have 'ranges' to be an MMIO address, and the
> > address cell size is set by the immediate parent.
> >
> >> Then if a device comes along that due to some twisted fabric logic needs
> >> nonposted nGnRnE mappings for PCIe (even though the actual PCIe ops will
> >> end up posted at the bus anyway)... how do we represent that? Declare
> >> that another "nonposted-mmio" on the PCIe bus means "no, really, use
> >> nonposted mmio for this"?
> >
> > If we're strict, yes. The PCI host bridge would have to have "nonposted-mmio".
>
> Works for me; then let's just make it non-recursive.
>
> Do you think we can get rid of the Apple-only optimization if we do
> this? It would mean only looking at the parent during address
> resolution, not recursing all the way to the top, so presumably the
> performance impact would be quite minimal.

Yeah, that should be fine. I'd keep an IS_ENABLED() config check
though. Then I'll also know if anyone else needs this.

Rob