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docs: driver-model: document driver_override

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Now that we support driver_override as a driver-core feature through
struct device and struct bus_type, add some documentation in the context
of how a device / driver binding is established.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://patch.msgid.link/20260303115720.48783-3-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
This commit is contained in:
Danilo Krummrich
2026-03-17 20:30:57 +01:00
parent cb3d1049f4
commit bcd085d5c7
1 changed files with 48 additions and 0 deletions
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Documentation/driver-api/driver-model/binding.rst
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@@ -99,3 +99,51 @@ of the driver is decremented. All symlinks between the two are removed.
When a driver is removed, the list of devices that it supports is
iterated over, and the driver's remove callback is called for each
one. The device is removed from that list and the symlinks removed.
Driver Override
~~~~~~~~~~~~~~~
Userspace may override the standard matching by writing a driver name to
a device's ``driver_override`` sysfs attribute. When set, only a driver
whose name matches the override will be considered during binding. This
bypasses all bus-specific matching (OF, ACPI, ID tables, etc.).
The override may be cleared by writing an empty string, which returns
the device to standard matching rules. Writing to ``driver_override``
does not automatically unbind the device from its current driver or
make any attempt to load the specified driver.
Buses opt into this mechanism by setting the ``driver_override`` flag in
their ``struct bus_type``::
const struct bus_type example_bus_type = {
...
.driver_override = true,
};
When the flag is set, the driver core automatically creates the
``driver_override`` sysfs attribute for every device on that bus.
The bus's ``match()`` callback should check the override before performing
its own matching, using ``device_match_driver_override()``::
static int example_match(struct device *dev, const struct device_driver *drv)
{
int ret;
ret = device_match_driver_override(dev, drv);
if (ret >= 0)
return ret;
/* Fall through to bus-specific matching... */
}
``device_match_driver_override()`` returns > 0 if the override matches
the given driver, 0 if the override is set but does not match, or < 0 if
no override is set at all.
Additional helpers are available:
- ``device_set_driver_override()`` - set or clear the override from kernel code.
- ``device_has_driver_override()`` - check whether an override is set.