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

[Liu Ying <victor.liu@nxp.com>]

On Fri, 2022-08-05 at 10:55 -0700, Saravana Kannan wrote:
> 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.

It seems that of_platform_default_populate() is _not_ the right
solution, because simple-bus/simple-mfd devices probed by this driver,
as child nodes of simple-pm-bus, are not power managed buses, which
means simple-bus/simple-mfd's child devices PM operations cannot be
propagated to simple-pm-bus.  I'm assuming that simple-bus/simple-mfd
devices probed by this driver should not be child nodes of simple-pm-
bus at all.

> 
> > 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?

I realized this propagation issue with this patch of mine when I looked
at Rob's comment - to use of_platform_default_populate() to cover
simple-bus. This propagation issue is the reason why I'm assuming
simple-bus/simple-mfd devices probed by this driver should not be child
nodes of simple-pm-bus at all.

> 
> > 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.

I don't attempt to remove any of the existing ONLY_BUS.

> 
> 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";

fsl,imx8qxp-csr.yaml[1](already upstreamed) mentions "fsl,imx8qm-lvds-
csr" and "fsl,imx8qxp-mipi-lvds-csr" compatible string entries. It
follows the SoC name(e.g., imx8qm) + subsytem name(e.g., lvds) + IP
name(csr) fashion, which exactly tells what the IP is. Although no real
device tree is using the IP yet, I tend not to change the compatible
string (DT maintainers usually don't like the change I'm assuming).

[1] Documentation/devicetree/bindings/mfd/fsl,imx8qxp-csr.yaml

> 
> 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 take a look at the examples in fsl,imx8qxp-csr.yaml[1], you'll
find that a "ipg" clock is syscon@56221000's property. Drivers[2][3]
for child nodes pxl2dpi/ldb call syscon_node_to_regmap() to get regmaps
from the syscon. The problem is that syscon_node_to_regmap() attaches
the "ipg" clock to the regmaps by calling device_node_get_regmap() with
"check_clk = true". Then, the clock will be managed by both the regmap
driver and the simple-pm-bus driver, thus unreasonable clock enable
count. I know drivers[2][3] may call device_node_to_regmap() instead
with "check_clk = false", but the drivers will be changed and they
don't really know which funtion should be called.

[2] drivers/gpu/drm/bridge/imx/imx8qxp-pxl2dpi.c
[3] drivers/gpu/drm/bridge/imx/imx-ldb-helper.c


And, like I mentioned, "fsl,imx8qm-lvds-csr" tells the exact IP ...

> 
> 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.

... and can be used by a dedicated driver - yes, I'm going to write a
driver for "fsl,imx8qm-lvds-csr". The driver probe function just
essentially calls pm_runtime_enable() and devm_of_platform_populate(),
that's it. Leave the "ipg" clock managed by the regmap driver.

Make sense?

> 
> Long story short, with what I understand so far, I think what you
> need
> to do are:
> 1. Patch to manage clock.

I'll do this, just like patch 2 does.

> 2. Patch to use of_platform_default_populate()

Nope for this one.
Based on my understanding, I won't use of_platform_default_populate()
in this driver to populate child nodes of simple-bus/simple-mfd devices
.

> 3. Fix up the compatible string to list simple-pm-bus in it.

Nope, I'm afraid I'm not willing to change the compatible string.

So, it looks like what I need to do are:
1. Drop this patch (patch 1).
2. Patch to manage clocks (patch 2).
3. Add a dedicated driver for "fsl,imx8qm-lvds-csr"/"fsl,imx8qxp-mipi-
lvds-csr".

Regards,
Liu Ying

> 
> > 
> > > 
> > > 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
> > > > >