Re: [PATCH v3 1/2] drivers: bus: simple-pm-bus: Add support for probing simple bus only devices

[Saravana Kannan <saravanak@google.com>]

On Mon, Sep 6, 2021 at 12:54 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> Hi Saravana,
>
> Thanks for your patch!
>
> CC linux-pm, Lee (mfd)
>
> On Sat, Sep 4, 2021 at 2:05 AM Saravana Kannan <saravanak@google.com> wrote:
> > fw_devlink could end up creating device links for bus only devices.
> > However, bus only devices don't get probed and can block probe() or
> > sync_state() [1] call backs of other devices. To avoid this, probe these
> > devices using the simple-pm-bus driver.
> >
> > However, there are instances of devices that are not simple buses (they
> > get probed by their specific drivers) that also list the "simple-bus"
> > (or other bus only compatible strings) in their compatible property to
> > automatically populate their child devices. We still want these devices
> > to get probed by their specific drivers. So, we make sure this driver
> > only probes devices that are only buses.
>
> Note that this can also be the case for buses declaring compatibility
> with "simple-pm-bus".  However, at the moment, none of such device
> nodes in upstream DTS files have device-specific drivers.

Not sure about mfd, but I want to make sure we don't confuse busses
(which are typically added to a class) with these "simple bus" devices
that are added to platform_bus. Also if these other buses are actually
causing an issue, then then should implement their own stub driver or
use try patch[2] if they are added to classes (devices on classes
don't probe)

[2] - https://lore.kernel.org/lkml/20210831224510.703253-1-saravanak@google.com/

>
> > [1] - https://lore.kernel.org/lkml/CAPDyKFo9Bxremkb1dDrr4OcXSpE0keVze94Cm=zrkOVxHHxBmQ@mail.gmail.com/
> > Signed-off-by: Saravana Kannan <saravanak@google.com>
> > Tested-by: Saravana Kannan <saravanak@google.com>
>
> > --- a/drivers/bus/simple-pm-bus.c
> > +++ b/drivers/bus/simple-pm-bus.c
> > @@ -13,11 +13,26 @@
> >  #include <linux/platform_device.h>
> >  #include <linux/pm_runtime.h>
> >
> > -
> >  static int simple_pm_bus_probe(struct platform_device *pdev)
> >  {
> > -       const struct of_dev_auxdata *lookup = dev_get_platdata(&pdev->dev);
> > -       struct device_node *np = pdev->dev.of_node;
> > +       const struct device *dev = &pdev->dev;
> > +       const struct of_dev_auxdata *lookup = dev_get_platdata(dev);
> > +       struct device_node *np = dev->of_node;
> > +       const struct of_device_id *match;
> > +
> > +       match = of_match_device(dev->driver->of_match_table, dev);
> > +
> > +       /*
> > +        * These are transparent bus devices (not simple-pm-bus matches) that
> > +        * have their child nodes populated automatically.  So, don't need to
> > +        * do anything more.
> > +        */
> > +       if (match && match->data) {
> > +               if (of_property_match_string(np, "compatible", match->compatible) == 0)
>
> Does this work as expected? Having multiple compatible values in a
> device node does not guarantee there exist a separate driver for any
> of the device-specific compatible values.

Right, and if they are platform devices that are equivalent to
simple-bus (meaning, they don't do anything in Linux and just have
their devices populated) we can add those to this list too.

>
> > +                       return 0;
> > +               else
> > +                       return -ENODEV;
>
> So if we get here, as both branches use "return", we skip the
> pm_runtime_enable() and of_platform_populate() below:
>   - of_platform_populate() is handled for these buses by
>     of_platform_default_populate(), so that's OK,
>   - I'm wondering if any of the simple-mfd sub-devices use Runtime
>     PM, but currently fail to save power because pm_runtime_enable()
>     is never called for the MFD container, just like with simple-bus...

But this doesn't affect simple-mfd though.

>
> > +       }
> >
> >         dev_dbg(&pdev->dev, "%s\n", __func__);
> >
> > @@ -31,14 +46,25 @@ static int simple_pm_bus_probe(struct platform_device *pdev)
> >
> >  static int simple_pm_bus_remove(struct platform_device *pdev)
> >  {
> > +       const void *data = of_device_get_match_data(&pdev->dev);
> > +
> > +       if (data)
> > +               return 0;
> > +
> >         dev_dbg(&pdev->dev, "%s\n", __func__);
> >
> >         pm_runtime_disable(&pdev->dev);
> >         return 0;
> >  }
> >
> > +#define ONLY_BUS       ((void *) 1) /* Match if the device is only a bus. */
> > +
> >  static const struct of_device_id simple_pm_bus_of_match[] = {
> >         { .compatible = "simple-pm-bus", },
> > +       { .compatible = "simple-bus",   .data = ONLY_BUS },
> > +       { .compatible = "simple-mfd",   .data = ONLY_BUS },
> > +       { .compatible = "isa",          .data = ONLY_BUS },
>
> #ifdef CONFIG_ARM_AMBA ?

Not needed? If CONFIG_ARM_AMBA isn't enabled, the device wouldn't be
created in the first place.

>
> > +       { .compatible = "arm,amba-bus", .data = ONLY_BUS },
> >         { /* sentinel */ }
>
> This is now (almost[*]) the same as of_default_bus_match_table[]
> in drivers/of/platform.c. Perhaps it can be shared?

I wanted this table to be expandable as needed. That's why I'm
intentionally not using of_default_bus_match_table[].

>
> [*] Especially if "simple-pm-bus" and "simple-bus" would be treated
>     the same.

They are not treated the same way.

-Saravana

>
> >  };
> >  MODULE_DEVICE_TABLE(of, simple_pm_bus_of_match);
>
> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds