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

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Message-ID: <c4ae5530acef6b737c2a6a12e4c25aa91b47d077.camel@nxp.com>
Date:   Tue, 15 Nov 2022 13:21:58 +0800
From:   Liu Ying <victor.liu@....com>
To:     Rob Herring <robh@...nel.org>
Cc:     Lee Jones <lee@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, shawnguo@...nel.org,
        s.hauer@...gutronix.de, kernel@...gutronix.de, festevam@...il.com,
        linux-imx@....com
Subject: Re: [PATCH RESEND] mfd: Add Freescale i.MX8qxp Control and Status
 Registers (CSR) module driver

On Mon, 2022-11-14 at 14:54 -0600, Rob Herring wrote:
> On Mon, Nov 7, 2022 at 9:58 PM Liu Ying <victor.liu@....com> wrote:
> > 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@....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:
> 
> As 'simple-bus' is for MMIO devices, so is 'simple-pm-bus' with the
> addition of PM capabilities. That means you have registers defined
> (reg), but you don't.
> 
> Either you have a block with mixed functions or you have separate
> blocks. If the register space is all mixed together, then it is the
> former and an MFD. Don't be designing your binding based on Linux
> behavior.

Thanks for clarifying how to differetiate MFD and 'simple-bus'/'simple-
pm-bus'.  I would say the register space of the CSR module is mixed
together, e.g., LVDS PHY child device has a register offset 0x00,
PXL2DPI child device has a register offset 0x40 and LDB child device
has register offsets 0x20 and 0xe0 in i.MX8qxp MIPI DSI/LVDS combo
subsystem CSR module register space.  So, it appears to be a MFD.

Lee, what do you think?  If it is indeed an MFD, a new MFD driver for
the CSR module is needed then.

Regards,
Liu Ying