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

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

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.  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. 

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.

The patch set to support i.MX8qxp pixel link MSI bus can be found at:

https://lore.kernel.org/lkml/20221017074039.4181843-1-victor.liu@nxp.com/

Regards,
Liu Ying