Re: [PATCH v5] gpiolib: Bind gpio_device to a driver to enable fw_devlink=on by default

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

On Wed, Jan 27, 2021 at 03:31:17PM +0100, Bartosz Golaszewski wrote:
> On Wed, Jan 27, 2021 at 3:22 PM Greg Kroah-Hartman
> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Fri, Jan 22, 2021 at 11:35:59AM -0800, Saravana Kannan 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.
> > >
> > > [1] - https://lore.kernel.org/lkml/20201014191235.7f71fcb4@xhacker.debian/
> > > [2] - https://lore.kernel.org/lkml/e28e1f38d87c12a3c714a6573beba6e1@xxxxxxxxxx/
> > > Cc: Marc Zyngier <maz@xxxxxxxxxx>
> > > Cc: Jisheng Zhang <Jisheng.Zhang@xxxxxxxxxxxxx>
> > > Cc: Kever Yang <kever.yang@xxxxxxxxxxxxxx>
> > > Fixes: e590474768f1 ("driver core: Set fw_devlink=on by default")
> >
> > As this commit is in my driver-core git tree, can I just take this in
> > the same tree?  Can I get an ack from the maintainer for this?
> >
> > thanks,
> >
> > greg k-h
> 
> Go ahead.
> 
> Acked-by: Bartosz Golaszewski <bgolaszewski@xxxxxxxxxxxx>

Thanks, now queued up.

greg k-h