Re: cxl/region.c improvements and DAX/Hotplug plumbing

[Gregory Price <gourry@gourry.net>]

On Wed, Jan 21, 2026 at 02:38:48PM -0500, Gregory Price wrote:
> --------------------------------
> Problem: Per-region usage policy
> --------------------------------
... snip ...
> 
>   ABI:  (RW) regionN/region_driver
>         Read: Displays what region driver is assigned
>         Write: Changing an uncommitted region's underlying driver
> 
>   ABI:  regionN/rctl/*
>         Exposes region_driver specific controls / information
>         example: auto-online policy for sysram_region
>  

Referencing this thread and questions from Ira:
https://lore.kernel.org/linux-cxl/aXGHgtAHNVWJsZbo@gourry-fedora-PF4VCD3F/

the appropriate design here is likely breaking out new drivers with
their own bind functions and leaving cxl/drivers/region/bind to be the
auto-decoder / compat interface.

so this turns into:

ABI:
   cxl/drivers/dax_region/bind
   cxl/drivers/pmem_region/bind
   cxl/drivers/sysram_region/bind

   Private regions likely remain internal interaces for device drivers
   (there's no way for userland to configure a set of callbacks)

   (Note for Dan: Each probe function here can determine which
      PARTMODE's are valid for that driver - so we can prevent
      pmem from ever using non-pmem drivers)

This implies some changes to dax_region to at least not immediately
probe the dax device on creation so that policy can be programmed.

(see: dax_bus_probe() - unconditionally probes at discovery)

So laying out the current workflow:

CURRENT: region/bind auto-decoder / compat workflow
----------------------------------------------------
echo region0 > decoder0.0/create_ram_region
   => creates region0
   /* program region (decoders, targets */

echo region0 > cxl/drivers/region/bind
  => probe() creates dax_region
  => dax_region creates, configures, and registers dev_dax
  => dax_bus_probe discovers dev_dax and selects a driver
     => IORESOURCE_DAX_KMEM = dax_kmem if KMEM built in
     => otherwise device_dax
  => dev_dax probe happens automatically from bus_probe()
     => if device_dax driver, make /dev/dax/* and stop
     => if kmem driver, engage memory-hotplug.c
        => system default hotplug policy is applied

All of this basically just happens automagically
----------------------------------------------------


The new workflows for manually created/programmed regions:

Manual dax_region Workflow
------------------------------------
echo region0 > decoder0.0/create_ram_region
   => creates region0
   /* program region0 (decoders, targets) */

echo region0 > cxl/drivers/dax_region/bind
   => creates dax_region
      => selects device_dax driver
   => creates unprobed dev_dax
   /* program dax_region controls */

echo daxN.M > dax/drivers/device_dax/bind
   => probes daxN.M
   => creates /dev/dax/ file
------------------------------------

Manual sysram_region Workflow
---------------------------------------
echo region0 > decoder0.0/create_ram_region
   => creates region0
   /* program region0 (decoders, targets) */

echo region0 > cxl/drivers/sysram_region/bind
   => creates sysram_region which
      => creates dax_region
         => creates unprobed dev_dax
      => selects dev_kmem driver for dev_dax

/* program hotplug policy */
echo online_movable > sysram_region/hotplug
   => dax_region.hotplug_mode = MMOP_ONLINE_MOVABLE

echo daxN.M > dax/drivers/kmem/bind
   => probe daxN.M
   => add_memory_driver_managed(..., dax_region.hotplug_mod);
---------------------------------------


And for dynamic capacity regions, you can use these same drivers, it
just changes the default behavior of [dax,sysram]_region when probed. 

Manual dc_region workflow
---------------------------------------
echo region0 > decoder0.0/create_dc_region
   => creates region0
   /* program region0 (decoders, targets) */

echo region0 > cxl/drivers/[dax, sysram]/bind
  => creates xxx_region WITHOUT dev_dax
  /* At this point, the extent discovery process takes over */

extent set arrives:
  => dc code calls `[dax,sysram]_region.add_extents(tag, extents)`
     => dax_region    -> create new device_dax w/ set
     => sysram_region -> create new dax_kmem w/ set
---------------------------------------


cxl-cli
---------------------------------------
What this should look like to cxl-cli is something like:

  cxl create-region -t [pmem,ram,dc] --driver=[pmem,dax,sysram,] ...
  /* if not --driver ... -c --controller ? */

And since `[dax,sysram,...]_region/bind` will restrict which PARTMODE is
valid (pmem=>[pmem],  ram=>[dax, sysram], dc=>[dax, sysram, ...]),

We have a clean failure condition that lets us undo the probe process
above if the user selects a bad combination.

   cxl create-region -t pmem --driver=sysram ...  => -ENOSUPP
----------------------------------------


I think this detangles most of region.c probe/policy issues.

From here most everything else I describe can be implemented in the
relevant driver directory. (replaces region0/rctl/ in original email)

   region0/sysram_region/*                         - policy controls
   region0/sysram_region/dax_region/*              - account daxN.M's
   region0/sysram_region/dax_region/daxN.M/hotplug - atomic hotplug

~Gregory