Re: [PATCH v3 1/3] drivers: bus: simple-pm-bus: Populate simple MFD child devices

[Saravana Kannan <saravanak@google.com>]

On Fri, Aug 5, 2022 at 3:07 AM Liu Ying <victor.liu@nxp.com> wrote:
>
> On Thu, 2022-08-04 at 11:26 -0700, Saravana Kannan wrote:
> > On Thu, Aug 4, 2022 at 5:18 AM Rob Herring <robh+dt@kernel.org>
> > wrote:
> > >
> > > On Thu, Aug 4, 2022 at 12:10 AM Liu Ying <victor.liu@nxp.com>
> > > wrote:
> > > >
> > > > There could be simple MFD device(s) connected to a simple PM bus
> > > > as child
> > > > node(s), like Freescale i.MX8qxp pixel link MSI bus. Add a child
> > > > match
> > > > table as an argument to of_platform_populate() function call to
> > > > specify
> > > > the simple MFD devices so that they can be populated.
> > >
> > > There could be a simple-bus under it as well. You should just use
> > > of_platform_default_populate() instead.
> >
> > I'm confused why we even need this patch. Wouldn't this driver
> > automatically probe simple-mfd buses and populate its child devices?
> > We already have it in simple_pm_bus_of_match.
>
> First of all, this driver doesn't populate simple-mfd bus's child
> devices because "ONLY_BUS" is set in simple_pm_bus_of_match[] for
> simple-mfd.
>
> The device tree I'm working with is something like this:
>
> bus@560000000 {
>         compatible = "fsl,aips-bus", "simple-bus";
>         ...
>
>         bus@562000000 {
>                 compatible = "fsl,imx8qm-display-pixel-link-msi-bus", "simple-
> pm-bus";
>                 ...
>
>                 syscon@56241000 {
>                         compatible = "fsl,imx8qm-lvds-csr", "syscon", "simple-mfd";
>                         ...
>
>                         syscon_child {};
>                 };
>
>                 /* more regular mmap devices */
>         };
> };
>
> IIUC, default buses listed in of_default_bus_match_table[], including
> simple-bus and simple-mfd, are populated by
> of_platform_default_populate() in a recursive fashion, when
> of_platform_default_populate_init() is called.  However, simple-pm-bus
> is not listed in that table.  So, bus@562000000 (simple-pm-bus) is the
> last one to be populated successfully and syscon@56241000 (simple-mfd)
> is not populated (recursion stops).

Ok, it's working as intended so far.

> Then, this patch adds a match table to populate syscon@56241000 (simple
> -mfd) _and_ it's child nodes when bus@562000000 (simple-pm-bus) is
> probed.  of_platform_populate() will populate syscon@56241000 (simple-
> mfd) and it's child devices (sycon_child) together. Hence, sycon_child
> devices will be probed ok.

I think of_platform_default_populate() is the right solution here
instead of spinning up a new table. Because a tree of simple-bus
children of simple-pm-bus would have the same problem you are facing
with simple-mfd's children not being populated.

> The problem is that syscon@56241000 (simple-mfd) fails to be probed
> with return code -ENODEV as "ONLY_BUS" is set and "simple-mfd" is the
> 3rd compatible string.

Ah, thanks for the example of your DT. My bad, I had forgotten the
"simple-mfd" is one of the default populate busses and can be a 2nd or
later entry in the compatible string and still have its children
populated by default by OF platform code.

> Even if it's probed ok, syscon@56241000 (simple-
> mfd) is not power managed, which means syscon_child devices' PM
> operations won't be propagated to bus@562000000 (simple-pm-bus) (?).
> Anyway, somehow, syscon_child devices do work, based on my test.

Aren't you seeing this propagation issue even with your current patches?

> With regard to PM, simple-bus is the same if it sits at simple-mfd's
> place.  So, maybe, simple-mfd and simple-bus should be power managed as
> well?  Or, simple-pm-bus should have no simple-mfd and simple-bus child
> nodes at all?

The problem is that there are cases of devices with real drivers that
also list simple-bus as their secondary compatible string. So we can't
really remove any of the existing ONLY_BUS as that could cause
simple-pm-bus driver to probe them instead of the real driver.

In your case, why even list this as "fsl,imx8qm-lvds-csr"? Can't you
just change your compatible string from:
"fsl,imx8qm-lvds-csr", "syscon", "simple-mfd";
To:
"simple-pm-bus", "syscon", "simple-mfd";

You are treating it exactly as a simple-pm-bus. So I don't see what
this extra "fsl,imx8qm-lvds-csr" distinction brings. Or make it if you
really want the "fsl,imx8qm-lvds-csr" in there:
"fsl,imx8qm-lvds-csr", "simple-pm-bus", "syscon", "simple-mfd";

If you are actually going to write a driver for "fsl,imx8qm-lvds-csr"
then you need to have that driver bind to this device of yours and do
PM management and populate the child devices if they aren't already.

Long story short, with what I understand so far, I think what you need
to do are:
1. Patch to manage clock.
2. Patch to use of_platform_default_populate()
3. Fix up the compatible string to list simple-pm-bus in it.

>
> >
> > I'm wondering if you are trying to workaround the behavior of having
> > "ONLY_BUS" set in simple_pm_bus_of_match for "simple-mfd". Have you
> > tried deleting that field and see if it does what you want?
>
> Without this patch, deleting "ONLY_BUS" works for me, as syscon_child
> devices are populated when syscon@56241000 (simple-mfd) is probed.
> Deleting "ONLY_BUS" may make simple-mfd a power managed device. Is it a
> right thing to do?

Ignore my point about deleting ONLY_BUS. That's wrong because then the
simple-pm-bus driver can end up probing any device that lists
simple-mfd even if there's another driver that could (like
"fsl,imx8qm-lvds-csr") and we don't want that.

-Saravana



>
> Regards,
> Liu Ying
>
> >
> > And we wouldn't need to use of_platform_default_populate() because
> > this driver would take care of doing that recursively. Especially
> > when
> > you need the clocks and power domain to be able to access the child
> > devices, you want the driver to probe and do that at each level
> > before
> > automatically recursively adding all the grand-children devices.
> >
> > -Saravana
> >
> > >
> > > >
> > > > Signed-off-by: Liu Ying <victor.liu@nxp.com>
> > > > ---
> > > > v1->v3:
> > > > * No change.
> > > >
> > > >  drivers/bus/simple-pm-bus.c | 7 ++++++-
> > > >  1 file changed, 6 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/bus/simple-pm-bus.c b/drivers/bus/simple-pm-
> > > > bus.c
> > > > index 6b8d6257ed8a..ff5f8ca5c024 100644
> > > > --- a/drivers/bus/simple-pm-bus.c
> > > > +++ b/drivers/bus/simple-pm-bus.c
> > > > @@ -13,6 +13,11 @@
> > > >  #include <linux/platform_device.h>
> > > >  #include <linux/pm_runtime.h>
> > > >
> > > > +static const struct of_device_id simple_pm_bus_child_matches[] =
> > > > {
> > > > +       { .compatible = "simple-mfd", },
> > > > +       {}
> > > > +};
> > > > +
> > > >  static int simple_pm_bus_probe(struct platform_device *pdev)
> > > >  {
> > > >         const struct device *dev = &pdev->dev;
> > > > @@ -49,7 +54,7 @@ static int simple_pm_bus_probe(struct
> > > > platform_device *pdev)
> > > >         pm_runtime_enable(&pdev->dev);
> > > >
> > > >         if (np)
> > > > -               of_platform_populate(np, NULL, lookup, &pdev-
> > > > >dev);
> > > > +               of_platform_populate(np,
> > > > simple_pm_bus_child_matches, lookup, &pdev->dev);
> > > >
> > > >         return 0;
> > > >  }
> > > > --
> > > > 2.25.1
> > > >
>