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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 v8 2/6] Documentation: ACPI: Document i2c-allow-low-power-probe _DSD property
Date: Thu,  3 Sep 2020 11:15:46 +0300
Message-ID: <20200903081550.6012-3-sakari.ailus@linux.intel.com> (raw)
In-Reply-To: <20200903081550.6012-1-sakari.ailus@linux.intel.com>

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/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


  parent reply index

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-03  8:15 [PATCH v8 0/6] Support running driver's probe for a device powered off Sakari Ailus
2020-09-03  8:15 ` [PATCH v8 1/6] i2c: Allow an ACPI driver to manage the device's power state during probe Sakari Ailus
2020-09-03  8:15 ` Sakari Ailus [this message]
2020-09-03  8:15 ` [PATCH v8 3/6] ACPI: Add a convenience function to tell a device is in low power state Sakari Ailus
     [not found]   ` <0c2d4728-6e32-ef93-8961-381fe36cfdaa@linux.intel.com>
2020-09-11 13:01     ` Sakari Ailus
2020-09-03  8:15 ` [PATCH v8 4/6] ov5670: Support probe whilst the device is in a " Sakari Ailus
2020-09-03  8:15 ` [PATCH v8 5/6] media: i2c: imx319: Support probe while the device is off Sakari Ailus
2020-09-03  8:15 ` [PATCH v8 6/6] at24: Support probing while off Sakari Ailus
2020-09-09  9:35   ` Bartosz Golaszewski
2020-09-09 11:11     ` Wolfram Sang
2020-09-09 11:56       ` Bartosz Golaszewski
2020-09-09 12:16         ` Sakari Ailus
2020-09-11 12:11         ` Sakari Ailus
2020-10-06 11:20   ` Tomasz Figa
2020-10-06 11:23     ` Tomasz Figa
2020-09-11 12:49 ` [PATCH v8 0/6] Support running driver's probe for a device powered off Luca Ceresoli
2020-09-11 13:01   ` Sakari Ailus
2020-09-14  7:58     ` Luca Ceresoli
     [not found]       ` <20200914094727.GM26842@paasikivi.fi.intel.com>
2020-09-14 16:49         ` Luca Ceresoli
2020-09-23 11:08           ` Sakari Ailus
2020-09-23 15:37             ` Luca Ceresoli
2020-09-26 12:38         ` Tomasz Figa
2020-09-27 19:39           ` Wolfram Sang
2020-09-27 19:44             ` Tomasz Figa
2020-09-28 14:17               ` Rafael J. Wysocki
2020-09-28 16:49                 ` Tomasz Figa
2020-09-28 20:49                   ` Sakari Ailus
2020-09-26 12:46 ` Tomasz Figa

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