On Tue, Sep 1, 2020 at 11:03 PM Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx> wrote: > > Document the probe-low-power _DSD property and how it is used with I²C > drivers. I would reorder the series to make this go right after the [1/6] or maybe even fold it into that patch. The point is that the changes in [1/6] clearly depend on the property defined here. > Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx> > --- > .../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. It would be good to refer to the document defining the generic _DSD properties mechanism and the GUID used for that from here. The meaning of "_DSD property" may not be entirely clear to the reader as it stands. And maybe call the property "i2c-allow-low-power-probe" or similar, to indicate that it is specific to i2c. > + > +How it works > +============ > + > +The property "allow-low-power-probe" boolean property may be used to tell Linux "boolean device property" ? > +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. It would be good to give an ASL example with this property provided. > 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 > --
