On Thu, Jan 29, 2026 at 08:50:30AM -0500, Bartosz Golaszewski wrote:
> On Thu, 29 Jan 2026 11:56:34 +0100, Laurent Pinchart said:
> > On Thu, Jan 29, 2026 at 10:11:46AM +0100, Bartosz Golaszewski wrote:
> >>
> >> For I2C both the problem is different (subsystem waiting forever for
> >> consumers to release all references) and the culprit: memory used to
> >> hold the reference-counted struct device is released the supplier
> >> unbind unconditionally. Unfortunately there's no way around it other
> >> than to first move it into a separate chunk managed by i2c core.
> >
> > Isn't there ? Can't the driver-specific data structure be
> > reference-counted instead of unconditionally freed at unbind time ?
>
> Oh, for sure, if we did from the start. But we did not and there are now
> hundreds of i2c drivers that do:
>
> struct my_i2c_drv_data {
> struct i2c_adapter adap;
> int my_other_drv_data;
> };
>
> and in probe:
>
> struct my_i2c_drv_data *data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
>
> (or just kzalloc() with kfree() in remove, it doesn't matter)
>
> and the ownership of that data belongs to the driver. There's no way we could
> address it now so the next best thing is to work towards moving the ownership
> of struct i2c_adapter to the i2c core and make it reference counted using the
> internal kobject of the associated struct device.
What I'm reading here is essentially that we rolled out devm_kzalloc()
too quickly without understanding the consequences, and it has spread so
much that it can't be fixed properly now, so we need to find a
workaround. And now we're trying to work around the problem by rolling
out a revocable API that has barely seen any testing, and is known to
have design issues. Does any one else see the irony ? :-)
--
Regards,
Laurent Pinchart
