On Tue, Jan 21, 2020 at 2:41 PM Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx> wrote: > > 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/probe-low-power.rst | 28 +++++++++++++++++++ > Documentation/firmware-guide/acpi/index.rst | 1 + > 2 files changed, 29 insertions(+) > create mode 100644 Documentation/firmware-guide/acpi/dsd/probe-low-power.rst > > diff --git a/Documentation/firmware-guide/acpi/dsd/probe-low-power.rst b/Documentation/firmware-guide/acpi/dsd/probe-low-power.rst > new file mode 100644 > index 0000000000000..e0343ffefe071 > --- /dev/null > +++ b/Documentation/firmware-guide/acpi/dsd/probe-low-power.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. The _DSD property > +"probe-low-power" has been defined for this purpose. Well, if you say "has been defined", you need to provide a pointer to the definition document (a specification of some sort or similar). > + > +How it works > +============ > + > +The property "probe-low-power" 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 in its struct i2c_driver.probe_low_power field and the > +"probe-low-power" 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..4e4ac675c0ff8 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/probe-low-power > enumeration > osi > method-customizing > -- > 2.20.1 >
