cxl: Defer dport allocation for switch ports

The current implementation enumerates the dports during the cxl_port
driver probe. Without an endpoint connected, the dport may not be
active during port probe. This scheme may prevent a valid hardware
dport id to be retrieved and MMIO registers to be read when an endpoint
is hot-plugged. Move the dport allocation and setup to behind memdev
probe so the endpoint is guaranteed to be connected.

In the original enumeration behavior, there are 3 phases (or 2 if no CXL
switches) for port creation. cxl_acpi() creates a Root Port (RP) from the
ACPI0017.N device. Through that it enumerates downstream ports composed
of ACPI0016.N devices through add_host_bridge_dport(). Once done, it
uses add_host_bridge_uport() to create the ports that enumerate the PCI
RPs as the dports of these ports. Every time a port is created, the port
driver is attached, cxl_switch_porbe_probe() is called and
devm_cxl_port_enumerate_dports() is invoked to enumerate and probe
the dports.

The second phase is if there are any CXL switches. When the pci endpoint
device driver (cxl_pci) calls probe, it will add a mem device and triggers
the cxl_mem_probe(). cxl_mem_probe() calls devm_cxl_enumerate_ports()
and attempts to discovery and create all the ports represent CXL switches.
During this phase, a port is created per switch and the attached dports
are also enumerated and probed.

The last phase is creating endpoint port which happens for all endpoint
devices.

The new sequence is instead of creating all possible dports at initial
port creation, defer port instantiation until a memdev beneath that
dport arrives. Introduce devm_cxl_create_or_extend_port() to centralize
the creation and extension of ports with new dports as memory devices
arrive. As part of this rework, switch decoder target list is amended
at runtime as dports show up.

While the decoders are allocated during the port driver probe,
The decoders must also be updated since previously they were setup when
all the dports are setup. Now every time a dport is setup per endpoint,
the switch target listing need to be updated with new dport. A
guard(rwsem_write) is used to update decoder targets. This is similar to
when decoder_populate_target() is called and the decoder programming
must be protected.

Also the port registers are probed the first time when the first dport
shows up. This ensures that the CXL link is established when the port
registers are probed.

[dj] Use ERR_CAST() (Jonathan)

Link: https://lore.kernel.org/linux-cxl/20250305100123.3077031-1-rrichter@amd.com/
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

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