[PATCH v5 6/6] Documentation: ACPI: Document allow-low-power-probe _DSD property

Sakari Ailus
2020-08-10T14:25:19Z

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

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
---
.../acpi/dsd/allow-low-power-probe.rst | 28 +++++++++++++++++++
Documentation/firmware-guide/acpi/index.rst | 1 +
2 files changed, 29 insertions(+)
create mode 100644 Documentation/firmware-guide/acpi/dsd/allow-low-power-probe.rst

diff --git a/Documentation/firmware-guide/acpi/dsd/allow-low-power-probe.rst b/Documentation/firmware-guide/acpi/dsd/allow-low-power-probe.rst
new file mode 100644
index 0000000000000..6fcc89162b898
--- /dev/null
+++ b/Documentation/firmware-guide/acpi/dsd/allow-low-power-probe.rst
@@ -0,0 +1,28 @@
+.. 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
+"allow-low-power-probe" that can be used for this purpose.
+
+How it works
+============
+
+The property "allow-low-power-probe" boolean property 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 "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.
diff --git a/Documentation/firmware-guide/acpi/index.rst b/Documentation/firmware-guide/acpi/index.rst
index ad3b5afdae77e..0070fcde9e669 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/allow-low-power-probe
enumeration
osi
method-customizing
--
2.20.1