On Thu, Mar 11, 2021 at 2:12 AM Arnd Bergmann <a...@kernel.org> wrote: > > On Wed, Mar 10, 2021 at 6:01 PM Rob Herring <r...@kernel.org> wrote: > > > > On Wed, Mar 10, 2021 at 1:27 AM Hector Martin <mar...@marcan.st> 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. > > Works for me. > > > Yeah, that should be fine. I'd keep an IS_ENABLED() config check > > though. Then I'll also know if anyone else needs this. > > Ok, makes sense. > > Conceptually, I'd like to then see a check that verifies that the > property is only set for nodes whose parent also has it set, since > that is how AXI defines it: A bus can wait for the ack from its > child node, or it can acknowledge the write to its parent early. > However, this breaks down as soon as a bus does the early ack: > all its children by definition use posted writes (as seen by the > CPU), even if they wait for stores that come from other masters. > > Does this make sense to you?
BTW, I don't think it's clear in this thread, but the current definition proposed for the spec[1] and schema is 'nonposted-mmio' is specific to 'simple-bus'. I like this restriction and we can expand where 'nonposted-mmio' is allowed later if needed. It's possible to express in json-schema, but I think it wouldn't be pretty. Json-schema is not great for expressing inter-property constraints and it gets worse if we're talking inter-node constraints. I'd like to define a way to do python snippets of code for something like this, but that's way down on the wish list. Rob [1] https://github.com/devicetree-org/devicetree-specification/pull/40
