From: Sakari Ailus <sakari.ailus@linux.intel.com>
To: linux-i2c@vger.kernel.org
Cc: Wolfram Sang <wsa@the-dreams.de>,
"Rafael J. Wysocki" <rafael@kernel.org>,
linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
rajmohan.mani@intel.com, Tomasz Figa <tfiga@chromium.org>,
Bartosz Golaszewski <bgolaszewski@baylibre.com>,
Bingbu Cao <bingbu.cao@intel.com>,
Chiranjeevi Rapolu <chiranjeevi.rapolu@intel.com>,
Hyungwoo Yang <hyungwoo.yang@intel.com>,
linux-media@vger.kernel.org
Subject: [PATCH v10 3/7] Documentation: ACPI: Document _DSE object usage for enum power state
Date: Fri, 5 Feb 2021 15:25:01 +0200 [thread overview]
Message-ID: <20210205132505.20173-4-sakari.ailus@linux.intel.com> (raw)
In-Reply-To: <20210205132505.20173-1-sakari.ailus@linux.intel.com>
Document the use of the _DSE object for setting desirable power state
during probe.
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
---
Documentation/firmware-guide/acpi/index.rst | 1 +
.../firmware-guide/acpi/low-power-probe.rst | 69 +++++++++++++++++++
2 files changed, 70 insertions(+)
create mode 100644 Documentation/firmware-guide/acpi/low-power-probe.rst
diff --git a/Documentation/firmware-guide/acpi/index.rst b/Documentation/firmware-guide/acpi/index.rst
index f72b5f1769fb2..d02712acccbc0 100644
--- a/Documentation/firmware-guide/acpi/index.rst
+++ b/Documentation/firmware-guide/acpi/index.rst
@@ -25,5 +25,6 @@ ACPI Support
acpi-lid
lpit
video_extension
+ low-power-probe
extcon-intel-int3496
intel-pmc-mux
diff --git a/Documentation/firmware-guide/acpi/low-power-probe.rst b/Documentation/firmware-guide/acpi/low-power-probe.rst
new file mode 100644
index 0000000000000..b96804d959a6c
--- /dev/null
+++ b/Documentation/firmware-guide/acpi/low-power-probe.rst
@@ -0,0 +1,69 @@
+.. 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.
+
+How it works
+============
+
+The _DSE (Device State for Enumeration) object that evaluates to integer 0 may
+be used to tell Linux the highest allowed D state for a device during probe. 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
+_DSE object evaluates to integer higher than the D state of the device, the
+device will not be powered on (put in D0 state) for probe.
+
+The D states and thus also the allowed values for _DSE are listed below. Refer
+to [1] for more information on device power states.
+
+.. code-block:: text
+
+ Number State Description
+ 0 D0 Device fully powered on
+ 1 D1
+ 2 D2
+ 3 D3hot
+ 4 D3cold Off
+
+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.
+
+References
+==========
+
+[1] https://uefi.org/specifications/ACPI/6.4/02_Definition_of_Terms/Definition_of_Terms.html#device-power-state-definitions
+
+Example
+=======
+
+An ASL example describing an ACPI device using _DSE object to tell Operating
+System the device should remain powered off during probe looks like this. Some
+objects not relevant from the example point of view have been omitted.
+
+.. code-block:: text
+
+ Device (CAM0)
+ {
+ Name (_HID, "SONY319A")
+ Name (_UID, Zero)
+ Name (_CRS, ResourceTemplate ()
+ {
+ I2cSerialBus(0x0020, ControllerInitiated, 0x00061A80,
+ AddressingMode7Bit, "\\_SB.PCI0.I2C0",
+ 0x00, ResourceConsumer)
+ })
+ Name (_DSE, 0, NotSerialized)
+ {
+ Return (0x4)
+ }
+ }
--
2.20.1
next prev parent reply other threads:[~2021-02-06 0:07 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-05 13:24 [PATCH v10 0/7] Support running driver's probe for a device powered off Sakari Ailus
2021-02-05 13:24 ` [PATCH v10 1/7] ACPI: scan: Obtain device's desired enumeration power state Sakari Ailus
2021-02-05 13:25 ` [PATCH v10 2/7] i2c: Allow an ACPI driver to manage the device's power state during probe Sakari Ailus
2021-02-09 21:04 ` Wolfram Sang
2021-02-10 20:46 ` Sakari Ailus
2021-02-10 22:27 ` Sakari Ailus
2021-02-12 7:18 ` Wolfram Sang
2021-02-05 13:25 ` Sakari Ailus [this message]
2021-02-06 0:56 ` [PATCH v10 3/7] Documentation: ACPI: Document _DSE object usage for enum power state Randy Dunlap
2021-02-08 8:01 ` Sakari Ailus
2021-02-08 15:52 ` Randy Dunlap
2021-02-05 13:25 ` [PATCH v10 4/7] ACPI: Add a convenience function to tell a device is in low " Sakari Ailus
2021-02-05 13:25 ` [PATCH v10 5/7] ov5670: Support probe whilst the device is in a " Sakari Ailus
2021-02-05 13:25 ` [PATCH v10 6/7] media: i2c: imx319: Support probe while the device is off Sakari Ailus
2021-02-05 13:25 ` [PATCH v10 7/7] at24: Support probing while off Sakari Ailus
2021-02-08 16:44 ` Bartosz Golaszewski
2021-02-08 16:54 ` Rafael J. Wysocki
2021-02-09 15:49 ` Bartosz Golaszewski
2021-02-09 16:23 ` Sakari Ailus
2021-02-09 16:42 ` Rafael J. Wysocki
2021-02-09 16:54 ` Sakari Ailus
2021-02-09 16:58 ` Rafael J. Wysocki
2021-02-10 8:41 ` Sakari Ailus
2021-02-10 12:26 ` Bartosz Golaszewski
2021-02-10 20:46 ` Sakari Ailus
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