Re: [PATCH v2] gpiolib: Bind gpio_device to a driver to enable fw_devlink=on by default

[Andy Shevchenko <andy.shevchenko@xxxxxxxxx>]

On Tue, Jan 26, 2021 at 8:17 PM Saravana Kannan <saravanak@xxxxxxxxxx> wrote:
> On Tue, Jan 26, 2021 at 1:40 AM Andy Shevchenko
> <andy.shevchenko@xxxxxxxxx> wrote:
> > On Friday, January 22, 2021, Saravana Kannan <saravanak@xxxxxxxxxx> wrote:

...

> >> Case 1: The driver for "foo" populates struct device for these gpio*

the struct

> >> nodes and then probes them using a driver that binds with "acme,bar".
> >> This driver for "acme,bar" then registers the gpio* nodes with gpiolib.
> >> This lines up with how DT nodes with the "compatible" property are
> >> typically converted to struct devices and then registered with driver
> >> core to probe them. This also allows the gpio* devices to hook into all
> >> the driver core capabilities like runtime PM, probe deferral,
> >> suspend/resume ordering, device links, etc.
> >>
> >> Case 2: The driver for "foo" doesn't populate struct devices for these
> >> gpio* nodes before registering them with gpiolib. Instead it just loops
> >> through its child nodes and directly registers the gpio* nodes with
> >> gpiolib.
> >>
> >> Drivers that follow case 2 cause problems with fw_devlink=on. This is
> >> because fw_devlink will prevent bazz from probing until there's a struct

prevent the bazz

> >> device that has gpio0 as its fwnode (because bazz lists gpio0 as a GPIO
> >> supplier). Once the struct device is available, fw_devlink will create a
> >> device link with gpio0 device as the supplier and bazz device as the
> >> consumer. After this point, since the gpio0 device will never bind to a
> >> driver, the device link will prevent bazz device from ever probing.
> >>
> >> Finding and refactoring all the instances of drivers that follow case 2
> >> will cause a lot of code churn and it is not something that can be done
> >> in one shot. In some instances it might not even be possible to refactor
> >> them cleanly. Examples of such instances are [1] [2].
> >>
> >> This patch works around this problem and avoids all the code churn by
> >> simply setting the fwnode of the gpio_device and creating a stub driver
> >> to bind to the gpio_device. This allows all the consumers to continue
> >> probing when the driver follows case 2.

...

> > Do we need to unregister it at __exit initcall?
> > What side effects would be of the stub driver presence on the GPIO bus? Any traverse on it will work as before?
>
> I checked. There is no __exit initcall.

You might have checked further out of curiosity, but yeah, I used the
attribute name while the initcall name is __exitcall().

-- 
With Best Regards,
Andy Shevchenko