PCI/NPEM: Add Native PCIe Enclosure Management support

Native PCIe Enclosure Management (NPEM, PCIe r6.1 sec 6.28) allows managing
LEDs in storage enclosures. NPEM is indication oriented and it does not
give direct access to LEDs. Although each indication *could* represent an
individual LED, multiple indications could also be represented as a single,
multi-color LED or a single LED blinking in a specific interval.  The
specification leaves that open.

Each enabled indication (capability register bit on) is represented as a
ledclass_dev which can be controlled through sysfs. For every ledclass
device only 2 brightness states are allowed: LED_ON (1) or LED_OFF (0).
This corresponds to the NPEM control register (Indication bit on/off).

Ledclass devices appear in sysfs as child devices (subdirectory) of PCI
device which has an NPEM Extended Capability and indication is enabled in
NPEM capability register. For example, these are LEDs created for pcieport
"10000:02:05.0" on my setup:

  leds/
  ├── 10000:02:05.0:enclosure:fail
  ├── 10000:02:05.0:enclosure:locate
  ├── 10000:02:05.0:enclosure:ok
  └── 10000:02:05.0:enclosure:rebuild

They can be also found in "/sys/class/leds" directory. The parent PCIe
device domain/bus/device/function address is used to guarantee uniqueness
across leds subsystem.

To enable/disable a "fail" indication, the "brightness" file can be edited:

  echo 1 > ./leds/10000:02:05.0:enclosure:fail/brightness
  echo 0 > ./leds/10000:02:05.0:enclosure:fail/brightness

PCIe r6.1, sec 7.9.19.2 defines the possible indications.

Multiple indications for same parent PCIe device can conflict and hardware
may update them when processing new request. To avoid issues, driver
refresh all indications by reading back control register.

This driver expects to be the exclusive NPEM extended capability manager.
It waits up to 1 second after imposing new request, it doesn't verify if
controller is busy before write, and it assumes the mutex lock gives
protection from concurrent updates.

If _DSM LED management is available, we assume the platform may be using
NPEM for its own purposes (see PCI Firmware Spec r3.3 sec 4.7), so the
driver does not use NPEM. A future patch will add _DSM support; an info
message notes whether NPEM or _DSM is being used.

NPEM is a PCIe extended capability so it should be registered in
pcie_init_capabilities() but it is not possible due to LED dependency.  The
parent pci_device must be added earlier for led_classdev_register() to be
successful. NPEM does not require configuration on kernel side, so it is
safe to register LED devices later.

Link: https://lore.kernel.org/r/20240904104848.23480-3-mariusz.tkaczyk@linux.intel.com
Suggested-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Stuart Hayes <stuart.w.hayes@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

Author: mtkaczyk

Documentation/ABI/testing/sysfs-bus-pci

@@ -500,3 +500,66 @@ Description:
 		console drivers from the device.  Raw users of pci-sysfs
 		resourceN attributes must be terminated prior to resizing.
 		Success of the resizing operation is not guaranteed.
+
+What:		/sys/bus/pci/devices/.../leds/*:enclosure:*/brightness
+What:		/sys/class/leds/*:enclosure:*/brightness
+Date:		August 2024
+KernelVersion:	6.12
+Description:
+		LED indications on PCIe storage enclosures which are controlled
+		through the NPEM interface (Native PCIe Enclosure Management,
+		PCIe r6.1 sec 6.28) are accessible as led class devices, both
+		below /sys/class/leds and below NPEM-capable PCI devices.
+
+		Although these led class devices could be manipulated manually,
+		in practice they are typically manipulated automatically by an
+		application such as ledmon(8).
+
+		The name of a led class device is as follows:
+		<bdf>:enclosure:<indication>
+		where:
+
+		- <bdf> is the domain, bus, device and function number
+		  (e.g. 10000:02:05.0)
+		- <indication> is a short description of the LED indication
+
+		Valid indications per PCIe r6.1 table 6-27 are:
+
+		- ok (drive is functioning normally)
+		- locate (drive is being identified by an admin)
+		- fail (drive is not functioning properly)
+		- rebuild (drive is part of an array that is rebuilding)
+		- pfa (drive is predicted to fail soon)
+		- hotspare (drive is marked to be used as a replacement)
+		- ica (drive is part of an array that is degraded)
+		- ifa (drive is part of an array that is failed)
+		- idt (drive is not the right type for the connector)
+		- disabled (drive is disabled, removal is safe)
+		- specific0 to specific7 (enclosure-specific indications)
+
+		Broadly, the indications fall into one of these categories:
+
+		- to signify drive state (ok, locate, fail, idt, disabled)
+		- to signify drive role or state in a software RAID array
+		  (rebuild, pfa, hotspare, ica, ifa)
+		- to signify any other role or state (specific0 to specific7)
+
+		Mandatory indications per PCIe r6.1 sec 7.9.19.2 comprise:
+		ok, locate, fail, rebuild. All others are optional.
+		A led class device is only visible if the corresponding
+		indication is supported by the device.
+
+		To manipulate the indications, write 0 (LED_OFF) or 1 (LED_ON)
+		to the "brightness" file. Note that manipulating an indication
+		may implicitly manipulate other indications at the vendor's
+		discretion. E.g. when the user lights up the "ok" indication,
+		the vendor may choose to automatically turn off the "fail"
+		indication. The current state of an indication can be
+		retrieved by reading its "brightness" file.
+
+		The PCIe Base Specification allows vendors leeway to choose
+		different colors or blinking patterns for the indications,
+		but they typically follow the IBPI standard. E.g. the "locate"
+		indication is usually presented as one or two LEDs blinking at
+		4 Hz frequency:
+		https://en.wikipedia.org/wiki/International_Blinking_Pattern_Interpretation

drivers/pci/Kconfig

@@ -143,6 +143,15 @@ config PCI_IOV
 
 	  If unsure, say N.
 
+config PCI_NPEM
+	bool "Native PCIe Enclosure Management"
+	depends on LEDS_CLASS=y
+	help
+	  Support for Native PCIe Enclosure Management. It allows managing LED
+	  indications in storage enclosures. Enclosure must support following
+	  indications: OK, Locate, Fail, Rebuild, other indications are
+	  optional.
+
 config PCI_PRI
 	bool "PCI PRI support"
 	select PCI_ATS

drivers/pci/Makefile

@@ -35,6 +35,7 @@ obj-$(CONFIG_XEN_PCIDEV_FRONTEND) += xen-pcifront.o
 obj-$(CONFIG_VGA_ARB)		+= vgaarb.o
 obj-$(CONFIG_PCI_DOE)		+= doe.o
 obj-$(CONFIG_PCI_DYNAMIC_OF_NODES) += of_property.o
+obj-$(CONFIG_PCI_NPEM)		+= npem.o
 
 # Endpoint library must be initialized before its users
 obj-$(CONFIG_PCI_ENDPOINT)	+= endpoint/