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

[Saravana Kannan <saravanak@xxxxxxxxxx>]

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:
>>
>> There are multiple instances of GPIO device tree nodes of the form:
>>
>> foo {
>>         compatible = "acme,foo";
>>         ...
>>
>>         gpio0: gpio0@xxxxxxxx {
>>                 compatible = "acme,bar";
>>                 ...
>>                 gpio-controller;
>>         };
>>
>>         gpio1: gpio1@xxxxxxxx {
>>                 compatible = "acme,bar";
>>                 ...
>>                 gpio-controller;
>>         };
>>
>>         ...
>> }
>>
>> bazz {
>>         my-gpios = <&gpio0 ...>;
>> }
>>
>> Case 1: The driver for "foo" populates struct device for these gpio*
>> 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
>> 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.

-Saravana