EDAC: Update documentation for the CXL memory patrol scrub control feature

`Documentation/ABI/testing/sysfs-edac-scrub`

`Documentation/ABI/testing/sysfs-edac-ecs`

Examples

--------

The usage takes the form shown in these examples:

1. CXL memory Patrol Scrub

The following are the use cases identified why we might increase the scrub rate.

- Scrubbing is needed at device granularity because a device is showing

  unexpectedly high errors.

- Scrubbing may apply to memory that isn't online at all yet. Likely this

  is a system wide default setting on boot.

- Scrubbing at a higher rate because the monitor software has determined that

  more reliability is necessary for a particular data set. This is called

  Differentiated Reliability.

1.1. Device based scrubbing

CXL memory is exposed to memory management subsystem and ultimately userspace

via CXL devices. Device-based scrubbing is used for the first use case

described in "Section 1 CXL Memory Patrol Scrub".

When combining control via the device interfaces and region interfaces,

"see Section 1.2 Region based scrubbing".

Sysfs files for scrubbing are documented in

`Documentation/ABI/testing/sysfs-edac-scrub`

1.2. Region based scrubbing

CXL memory is exposed to memory management subsystem and ultimately userspace

via CXL regions. CXL Regions represent mapped memory capacity in system

physical address space. These can incorporate one or more parts of multiple CXL

memory devices with traffic interleaved across them. The user may want to control

the scrub rate via this more abstract region instead of having to figure out the

constituent devices and program them separately. The scrub rate for each device

covers the whole device. Thus if multiple regions use parts of that device then

requests for scrubbing of other regions may result in a higher scrub rate than

requested for this specific region.

Region-based scrubbing is used for the third use case described in

"Section 1 CXL Memory Patrol Scrub".

Userspace must follow below set of rules on how to set the scrub rates for any

mixture of requirements.

1. Taking each region in turn from lowest desired scrub rate to highest and set

   their scrub rates. Later regions may override the scrub rate on individual

   devices (and hence potentially whole regions).

2. Take each device for which enhanced scrubbing is required (higher rate) and

   set those scrub rates. This will override the scrub rates of individual devices,

   setting them to the maximum rate required for any of the regions they help back,

   unless a specific rate is already defined.

Sysfs files for scrubbing are documented in

`Documentation/ABI/testing/sysfs-edac-scrub`

2. CXL memory Error Check Scrub (ECS)

The Error Check Scrub (ECS) feature enables a memory device to perform error

checking and correction (ECC) and count single-bit errors. The associated

memory controller sets the ECS mode with a trigger sent to the memory

device. CXL ECS control allows the host, thus the userspace, to change the

attributes for error count mode, threshold number of errors per segment

(indicating how many segments have at least that number of errors) for

reporting errors, and reset the ECS counter. Thus the responsibility for

initiating Error Check Scrub on a memory device may lie with the memory

controller or platform when unexpectedly high error rates are detected.

Sysfs files for scrubbing are documented in

`Documentation/ABI/testing/sysfs-edac-ecs`