[PATCH v8 2/6] Documentation: ACPI: Document i2c-allow-low-power-probe _DSD property

[Sakari Ailus]

Document the probe-low-power _DSD property and how it is used with I²C
drivers.

Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
---
 .../acpi/dsd/i2c-allow-low-power-probe.rst    | 60 +++++++++++++++++++
 Documentation/firmware-guide/acpi/index.rst   |  1 +
 2 files changed, 61 insertions(+)
 create mode 100644 Documentation/firmware-guide/acpi/dsd/i2c-allow-low-power-probe.rst

diff --git a/Documentation/firmware-guide/acpi/dsd/i2c-allow-low-power-probe.rst b/Documentation/firmware-guide/acpi/dsd/i2c-allow-low-power-probe.rst
new file mode 100644
index 0000000000000..7e7ed3ad3ac69
--- /dev/null
+++ b/Documentation/firmware-guide/acpi/dsd/i2c-allow-low-power-probe.rst
@@ -0,0 +1,60 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======================================
+Probing I²C devices in low power state
+======================================
+
+Introduction
+============
+
+In some cases it may be preferred to leave certain devices powered off for the
+entire system bootup if powering on these devices has adverse side effects,
+beyond just powering on the said device. Linux recognizes the _DSD property [1]
+"i2c-allow-low-power-probe" that can be used for this purpose.
+
+How it works
+============
+
+The boolean device property "i2c-allow-low-power-probe" may be used to tell
+Linux that the I²C framework should instruct the kernel ACPI framework to leave
+the device in the low power state. If the driver indicates its support for this
+by setting the I2C_DRV_FL_ALLOW_LOW_POWER_PROBE flag in struct i2c_driver.flags
+field and the "i2c-allow-low-power-probe" property is present, the device will
+not be powered on for probe.
+
+The downside is that as the device is not powered on, even if there's a problem
+with the device, the driver likely probes just fine but the first user will
+find out the device doesn't work, instead of a failure at probe time. This
+feature should thus be used sparingly.
+
+Example
+=======
+
+An ASL example describing an ACPI device using this property looks like
+this. Some objects not relevant from the example point of view have been
+omitted.
+
+	Device (CAM0)
+        {
+		Name (_HID, "SONY319A")
+		Name (_UID, Zero)
+		Name (_CRS, ResourceTemplate ()
+		{
+			I2cSerialBus(0x0020, ControllerInitiated, 0x00061A80,
+				     AddressingMode7Bit, "\\_SB.PCI0.I2C0",
+				     0x00, ResourceConsumer)
+		})
+		Name (PRT4, Package() {
+			ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
+			Package () {
+				Package () { "i2c-allow-low-power-probe", 1 },
+			}
+                })
+	}
+
+References
+==========
+
+[1] Device Properties UUID For _DSD.
+    https://www.uefi.org/sites/default/files/resources/_DSD-device-properties-UUID.pdf,
+    referenced 2020-09-02.
diff --git a/Documentation/firmware-guide/acpi/index.rst b/Documentation/firmware-guide/acpi/index.rst
index ad3b5afdae77e..1f87b7072d1e6 100644
--- a/Documentation/firmware-guide/acpi/index.rst
+++ b/Documentation/firmware-guide/acpi/index.rst
@@ -11,6 +11,7 @@ ACPI Support
    dsd/graph
    dsd/data-node-references
    dsd/leds
+   dsd/i2c-allow-low-power-probe
    enumeration
    osi
    method-customizing
-- 
2.20.1