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

[Geert Uytterhoeven <geert@linux-m68k.org>]

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.

> [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.

> +                       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...

> +       }
>
>         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 ?

> +       { .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?

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

>  };
>  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