linux-kernel - Re: [PATCH v4 1/1] mfd: Add I2C based System Configuaration (SYSCON) access

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Date:   Thu, 2 Jul 2020 07:54:12 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Michael Walle <michael@...le.cc>
Cc:     robh+dt@...nel.org, broonie@...nel.org, gregkh@...uxfoundation.org,
        andriy.shevchenko@...ux.intel.com, devicetree@...r.kernel.org,
        linus.walleij@...aro.org, bgolaszewski@...libre.com, arnd@...db.de,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/1] mfd: Add I2C based System Configuaration (SYSCON)
 access

On Wed, 01 Jul 2020, Michael Walle wrote:

> Am 2020-07-01 09:04, schrieb Lee Jones:
> > On Wed, 01 Jul 2020, Michael Walle wrote:
> > 
> > > Hi Lee,
> > > 
> > > Am 2020-06-30 11:16, schrieb Michael Walle:
> > > > I'm just trying to use this for my sl28 driver. Some remarks, see below.
> > > >
> > > > Am 2020-06-22 09:51, schrieb Lee Jones:
> > > > > The existing SYSCON implementation only supports MMIO (memory mapped)
> > > > > accesses, facilitated by Regmap.  This extends support for registers
> > > > > held behind I2C busses.
> > > > >
> > > > > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> > > > > ---
> > > > > Changelog:
> > > > >
> > > > > v3 => v4
> > > > >   - Add ability to provide a non-default Regmap configuration
> > > > >
> > > > > v2 => v3
> > > > >   - Change 'is CONFIG' present check to include loadable modules
> > > > >     - s/#ifdef CONFIG_MFD_SYSCON_I2C/#if
> > > > > IS_ENABLED(CONFIG_MFD_SYSCON_I2C)/
> > > > >
> > > > > v1 => v2
> > > > >   - Remove legacy references to OF
> > > > >   - Allow building as a module (fixes h8300 0-day issue)
> > > > >
> > > > > drivers/mfd/Kconfig            |   7 +++
> > > > >  drivers/mfd/Makefile           |   1 +
> > > > >  drivers/mfd/syscon-i2c.c       | 104
> > > > > +++++++++++++++++++++++++++++++++
> > > > >  include/linux/mfd/syscon-i2c.h |  36 ++++++++++++
> > > > >  4 files changed, 148 insertions(+)
> > > > >  create mode 100644 drivers/mfd/syscon-i2c.c
> > > > >  create mode 100644 include/linux/mfd/syscon-i2c.h

[...]

> > > > This way, (a) a driver doesn't have to use "#include <linux/i2c.h>" just
> > > > to call to_i2c_client() (or i2c_verify_client()) and (b) you won't do it
> > > > all over again in all sub drivers.
> > > >
> > > > So you could just do a
> > > >   regmap = syscon_i2c_to_regmap(pdev->dev.parent);
> > > >
> > > > I've also noticed that the mmio syscon uses device_node as parameter.
> > > > What
> > > > was the reason to divert from that? Just curious.
> > > 
> > > How is this supposed to be used?
> > > 
> > > I had something like the following in mind:
> > > 
> > > &i2c {
> > >   cpld@4a {
> > >     compatible = "simple-mfd";
> > >     reg = <0x4a>;
> > > 
> > >     gpio@4 {
> > >       compatible = "vendor,gpio";
> > >       reg = <0x4>;
> > >     };
> > >   };
> > > };
> > 
> > Yes, that was the idea.
> > 
> > > But I think the childen are not enumerated if its an I2C device. And
> > > the actual i2c driver is also missing.
> > 
> > What do you mean?  Can you elaborate?
> 
> There is no i2c_driver instance who would create the regmap.

The regmap is created by the first caller of:

 syscon_i2c_to_regmap{_config}()

> If I'm
> reading the I2C code correctly, it won't probe any i2c device of a
> bus if there is no i2c_driver with an associated .probe() or
> .probe_new().

Why wouldn't the children be registered using i2c_driver?

> And even if it is probed, its subnodes won't be
> enumerated; the "simple-mfd" code only works for MMIO busses, right?
> Or I'm getting something really wrong here..

Then how are these I2C based devices able to call of_platform_populate()?

 drivers/mfd/gateworks-gsc.c
 drivers/mfd/lochnagar-i2c.c
 drivers/mfd/palmas.c
 drivers/mfd/smsc-ece1099.c
 drivers/mfd/stpmic1.c
 drivers/mfd/twl-core.c

Might require some more research into where your use-case is breaking
down.

-- 
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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