Re: [PATCH RESEND] mfd: Add Freescale i.MX8qxp Control and Status Registers (CSR) module driver

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

Hi Lee,

On Mon, 2022-11-07 at 09:05 +0000, Lee Jones wrote:
> On Wed, 02 Nov 2022, Liu Ying wrote:
> 
> > Hi Lee,
> > 
> > On Tue, 2022-11-01 at 13:53 +0800, Liu Ying wrote:
> > > Hi Lee,
> > > 
> > > On Mon, 2022-10-31 at 15:40 +0000, Lee Jones wrote:
> > > > On Mon, 17 Oct 2022, Liu Ying wrote:
> > > > 
> > > > > Freescale i.MX8qxp Control and Status Registers (CSR) module is a
> > > > > system
> > > > > controller. It represents a set of miscellaneous registers of a
> > > > > specific
> > > > > subsystem. It may provide control and/or status report interfaces
> > > > > to a
> > > > > mix of standalone hardware devices within that subsystem.
> > > > > 
> > > > > The CSR module in i.MX8qm/qxp SoCs is a child node of a simple
> > > > > power-managed
> > > > > bus(i.MX8qxp pixel link MSI bus). To propagate power management
> > > > > operations
> > > > > of the CSR module's child devices to that simple power-managed
> > > > > bus, add a
> > > > > dedicated driver for the CSR module. Also, the driver would
> > > > > populate the CSR
> > > > > module's child devices.
> > > > > 
> > > > > Signed-off-by: Liu Ying <victor.liu@nxp.com>
> > > > > ---
> > > > > The Freescale i.MX8qxp CSR DT bindings is at
> > > > > Documentation/devicetree/bindings/mfd/fsl,imx8qxp-csr.yaml.
> > > > > 
> > > > > Resend the patch based on v6.1-rc1.
> > > > > 
> > > > >  drivers/mfd/Kconfig           | 10 +++++++
> > > > >  drivers/mfd/Makefile          |  1 +
> > > > >  drivers/mfd/fsl-imx8qxp-csr.c | 53
> > > > > +++++++++++++++++++++++++++++++++++
> > > > >  3 files changed, 64 insertions(+)
> > > > >  create mode 100644 drivers/mfd/fsl-imx8qxp-csr.c
> > > 
> > > [...]
> > > 
> > > > > diff --git a/drivers/mfd/fsl-imx8qxp-csr.c b/drivers/mfd/fsl-
> > > > > imx8qxp-csr.c
> > > > > new file mode 100644
> > > > > index 000000000000..3915d3d6ca65
> > > > > --- /dev/null
> > > > > +++ b/drivers/mfd/fsl-imx8qxp-csr.c
> > > > > @@ -0,0 +1,53 @@
> > > > > +// SPDX-License-Identifier: GPL-2.0+
> > > > > +
> > > > > +/*
> > > > > + * Copyright 2022 NXP
> > > > > + */
> > > > > +
> > > > > +#include <linux/module.h>
> > > > > +#include <linux/of_platform.h>
> > > > > +#include <linux/platform_device.h>
> > > > > +#include <linux/pm_runtime.h>
> > > > > +
> > > > > +static int imx8qxp_csr_probe(struct platform_device *pdev)
> > > > > +{
> > > > > +	int ret;
> > > > > +
> > > > > +	pm_runtime_enable(&pdev->dev);
> > > > > +
> > > > > +	ret = devm_of_platform_populate(&pdev->dev);
> > > > 
> > > > The use of this API does not constitute a MFD.
> > > > 
> > > > Please use "simple-mfd" instead.
> > > 
> > > simple-mfd devices have "ONLY_BUS" set in simple-pm-bus.c, so the
> > > simple-pm-bus driver would not populate child devices of simple-mfd
> > > devices.
> 
> Right, simple-pm-bus will not populate child devices, because:

simple-pm-bus.c may populate child devices of simple-pm-bus devices
because "ONLY_BUS" is _not_ set for simple-pm-bus devices.

simple-pm-bus.c would _not_ populate child devices of simple-mfd
devices because "ONLY_BUS" is set for simple-mfd devices.

> 
>   /*                                                                     
>    * 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. We only match with the device if this driver is   
>    * the most specific match because we don't want to incorrectly bind to
>    * a device that has a more specific driver.                           
>    */                                                                    
> 
> So "simple-mfd" must be populated elsewhere i.e. drivers/of/platform.c.

If simple-mfd device is a child device of one device listed in
of_default_bus_match_table[], then it may be populated by
drivers/of/platform.c.  But, in my case, simple-mfd device is a child
device of simple-pm-bus device(not listed in that table), so it will
not be populated by drivers/of/platform.c.  Instead,
drivers/bus/simple-pm-bus.c would populate the simple-mfd device.

> 
> > > Also, the simple-pm-bus driver would not enable runtime
> > > power management for simple-mfd devices due to "ONLY_BUS", which
> > > means it would not propagate power management operations from child
> > > devices of simple-mfd devices to parent devices of simple-mfd
> > > devices.  That's why a dedicated fsl-imx8qxp-csr driver is needed. 
> 
> This is more of an issue.
> 
> Why can't this device use "simple-pm-bus" to have support for both
> auto-registration AND Runtime PM?

If I change the compatible string of the CSR module from
"fsl,imx8qxp-mipi-lvds-csr", "syscon", "simple-mfd"
to
"fsl,imx8qxp-mipi-lvds-csr", "syscon", "simple-pm-bus",
all devices I'm interested in are populated and all relevant drivers
can probe.  But, this change makes 'make dt_binding_check' for the
existing fsl,imx8qxp-csr.yaml fail:

--------------------------------8<------------------------------------
/kernel/linux/Documentation/devicetree/bindings/mfd/fsl,imx8qxp-
csr.example.dtb: syscon@56221000: $nodename:0: 'syscon@56221000' does
not match '^bus(@[0-9a-f]+)?$'
	From schema:
/kernel/linux/Documentation/devicetree/bindings/bus/simple-pm-bus.yaml
/kernel/linux/Documentation/devicetree/bindings/mfd/fsl,imx8qxp-
csr.example.dtb: syscon@56221000: '#address-cells' is a required
property
	From schema:
/kernel/linux/Documentation/devicetree/bindings/bus/simple-pm-bus.yaml
/kernel/linux/Documentation/devicetree/bindings/mfd/fsl,imx8qxp-
csr.example.dtb: syscon@56221000: '#size-cells' is a required property
	From schema:
/kernel/linux/Documentation/devicetree/bindings/bus/simple-pm-bus.yaml
/kernel/linux/Documentation/devicetree/bindings/mfd/fsl,imx8qxp-
csr.example.dtb: syscon@56221000: 'ranges' is a required property
	From schema:
/kernel/linux/Documentation/devicetree/bindings/bus/simple-pm-bus.yaml
--------------------------------8<------------------------------------

The error log basically complains two things:
1) The example nodename 'syscon@56221000' should match
'^bus(@[0-9a-f]+)?$'.
2) Missing '#address-cells', '#size-cells' and 'ranges' properties as
required by simple-pm-bus.

Regarding 1), if I change 'syscon@56221000' to 'bus@56221000', then the
below error comes along:
--------------------------------8<------------------------------------
/kernel/linux/Documentation/devicetree/bindings/mfd/fsl,imx8qxp-
csr.example.dtb: bus@56221000: $nodename:0: 'bus@56221000' does not
match '^syscon@[0-9a-f]+$'
	From schema:
/kernel/linux/Documentation/devicetree/bindings/mfd/fsl,imx8qxp-
csr.yaml
--------------------------------8<------------------------------------
So, it looks like "syscon" and "simple-pm-bus" can not be in compatbile
string at the same time.  Note that "syscon" is needed because other
device nodes may reference the CSR module through a phandle, like the
"fsl,imx8qxp-mipi-dphy" device.

Regarding 2), I may try to add those required properties, but it would
break the existing schemas of the child devices of the CSR module, like
the "fsl,imx8qxp-ldb" device, because "reg" property is not allowed.

So, it looks like the CSR module still should be simple-mfd device but
not simple-pm-bus device, right?

> 
> > One more point which might be overlooked - as mentioned in commit
> > message, the CSR module is a child node of a simple power-managed
> > bus(i.MX8qxp pixel link MSI bus), which means the child devices of the
> > CSR module(as a simple-mfd device) won't be populated by
> > of_platform_default_populate() from of_platform_default_populate_init()
> > because "simple-pm-bus" is not listed in of_default_bus_match_table[]
> > and hence recursion of of_platform_bus_create() will stop at the
> > simple-pm-bus. This is also a reason why a dedicated fsl-imx8qxp-csr
> > driver is needed to populated those child devices of the CSR module.
> 
> Not sure I know the semantics well enough (anymore) to get a
> meaningful picture of what you're trying to explain, and I do not have
> any suitable H/W here to mock-up a real-world test-bed / concept
> demonstrator to debug this for you.

I understand you have no hardware to debug this directly.  But, the
example in dt-binding doc for the i.MX8qxp pixel link MSI bus(a simple-
pm-bus) may give you a kinda full picture of what the relevant devices
look like.  I mentioned the patch set to add the MSI bus previously,
however, you may find the binding doc directly here -
https://lore.kernel.org/lkml/20221017074039.4181843-3-victor.liu@nxp.com/

> 
> The long and the short of it is; we have a bunch of automatic
> child-device-registering helpers in the kernel.  One of the mechanisms
> is bound to work for you if you structure your code appropriately.
> 
> Introducing an empty, meaningless driver, simply to call a single
> function it not acceptable.  Please make the infrastructure already
> offered specifically to solve this category of issue work for your
> use-case.

Yeah, I tried to not to introduce a new driver for the CSR module, but
it seems that it is needed.

Regards,
Liu Ying