Re: [PATCH v2 17/28] cxl/region: Add region creation support

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Thu, 14 Jul 2022 17:02:19 -0700
Dan Williams <dan.j.williams@intel.com> wrote:

> From: Ben Widawsky <bwidawsk@kernel.org>
> 
> CXL 2.0 allows for dynamic provisioning of new memory regions (system
> physical address resources like "System RAM" and "Persistent Memory").
> Whereas DDR and PMEM resources are conveyed statically at boot, CXL
> allows for assembling and instantiating new regions from the available
> capacity of CXL memory expanders in the system.
> 
> Sysfs with an "echo $region_name > $create_region_attribute" interface
> is chosen as the mechanism to initiate the provisioning process. This
> was chosen over ioctl() and netlink() to keep the configuration
> interface entirely in a pseudo-fs interface, and it was chosen over
> configfs since, aside from this one creation event, the interface is
> read-mostly. I.e. configfs supports cases where an object is designed to
> be provisioned each boot, like an iSCSI storage target, and CXL region
> creation is mostly for PMEM regions which are created usually once
> per-lifetime of a server instance. This is an improvement over nvdimm
> that pre-created "seed" devices that tended to confuse users looking to
> determine which devices are active and which are idle.
> 
> Recall that the major change that CXL brings over previous persistent
> memory architectures is the ability to dynamically define new regions.
> Compare that to drivers like 'nfit' where the region configuration is
> statically defined by platform firmware.
> 
> Regions are created as a child of a root decoder that encompasses an
> address space with constraints. When created through sysfs, the root
> decoder is explicit. When created from an LSA's region structure a root
> decoder will possibly need to be inferred by the driver.
> 
> Upon region creation through sysfs, a vacant region is created with a
> unique name. Regions have a number of attributes that must be configured
> before the region can be bound to the driver where HDM decoder program
> is completed.
> 
> An example of creating a new region:
> 
> - Allocate a new region name:
> region=$(cat /sys/bus/cxl/devices/decoder0.0/create_pmem_region)
> 
> - Create a new region by name:
> while
> region=$(cat /sys/bus/cxl/devices/decoder0.0/create_pmem_region)
> ! echo $region > /sys/bus/cxl/devices/decoder0.0/create_pmem_region
> do true; done
> 
> - Region now exists in sysfs:
> stat -t /sys/bus/cxl/devices/decoder0.0/$region
> 
> - Delete the region, and name:
> echo $region > /sys/bus/cxl/devices/decoder0.0/delete_region
> 
> Signed-off-by: Ben Widawsky <bwidawsk@kernel.org>
> [djbw: simplify locking, reword changelog]
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
V1 comments all addressed so
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>