Dave Jiang wrote:
On 11/4/2022 11:59 AM, Dan Williams wrote:
Verma, Vishal L wrote:
On Thu, 2022-11-03 at 17:30 -0700, Dan Williams wrote:
When programming port decode targets, the algorithm wants to ensure that two devices are compatible to be programmed as peers beneath a given port. A compatible peer is a target that shares the same dport, and where that target's interleave position also routes it to the same dport. Compatibility is determined by the device's interleave position being >= to distance. For example, if a given dport can only map every Nth position then positions less than N away from the last target programmed are incompatible.
The @distance for the host-bridge-cxl_port a simple dual-ported
^Is this meant to be "the distance for the host-bridge to a cxl_port"?
No, but I will preface this explanation by admitting "distance" may not be the best term for what this value is describing. CXL decode routes to targets in a round robin fashion per-port. Take the diagram below:
┌───────────────────────────────────┬──┐ │WINDOW0 │x2│ └─────────┬─────────────────┬───────┴──┘ │ │ ┌─────────▼────┬──┐ ┌──────▼───────┬──┐ │HB0 │x2│ │HB1 │x2│ └──┬────────┬──┴──┘ └─┬─────────┬──┴──┘ │ │ │ │ ┌──▼─┬──┐ ┌─▼──┬──┐ ┌─▼──┬──┐ ┌─▼──┬──┐ │DEV0│x4│ │DEV1│x4│ │DEV2│x4│ │DEV3│x4│ └────┴──┘ └────┴──┘ └────┴──┘ └────┴──┘ 0 2 1 3
...where an x4 region is being established, and all the xN values are the interleave-ways settings for those ports/devices. The @distance value for that "HB0" port is 2. I.e. in order for 2 devices in that region to be mapped under HB0 they must be at position X and X+2 in the region. The algorithm needs to be flexible to also allow this configuration:
┌───────────────────────────────────┬──┐ │WINDOW0 │x2│ └─────────┬─────────────────┬───────┴──┘ │ │ ┌─────────▼────┬──┐ ┌──────▼───────┬──┐ │HB0 │x2│ │HB1 │x2│ └──┬────────┬──┴──┘ └─┬─────────┬──┴──┘ │ │ │ │ ┌──▼─┬──┐ ┌─▼──┬──┐ ┌─▼──┬──┐ ┌─▼──┬──┐ │DEV3│x4│ │DEV2│x4│ │DEV1│x4│ │DEV0│x4│ └────┴──┘ └────┴──┘ └────┴──┘ └────┴──┘ 3 1 2 0
...and the algorithm can not know that a device is in the wrong position until trying to map the peer (like DEV0 and DEV1 are peers) into the decode. So "The @distance for the host-bridge-cxl_port" is referring to the value "2" for host-bridge-cxl_port:HB0 and host-bridge-cxl_port:HB1 in the diagram.
Also missing '/in/ a simple dual-ported..'?
Yes to this fixup though.
host-bridge configuration with 2 direct-attached devices is 1. An x2 region divded by 2 dports to reach 2 region targets.
The second sentence seems slightly incomprehensible too.
Oh, I think I meant that to be s/. An/, i.e. an/:
"...host-bridge configuration with 2 direct-attached devices is 1, i.e. an x2 region divded by 2 dports to reach 2 region positions"
s/divded/divided/ ?
Yup, thanks.