Pankaj,

On Tue, Sep 30, 2014 at 1:35 AM, Pankaj Dubey [off-list ref] wrote:

Currently a syscon entity can be only registered directly through a
platform device that binds to a dedicated syscon driver. However in
certain use cases it is desirable to make a device used with another
driver a syscon interface provider.

For example, certain SoCs (e.g. Exynos) contain system controller
blocks which perform various functions such as power domain control,
CPU power management, low power mode control, but in addition contain
certain IP integration glue, such as various signal masks,
coprocessor power control, etc. In such case, there is a need to have
a dedicated driver for such system controller but also share registers
with other drivers. The latter is where the syscon interface is helpful.

In case of DT based platforms, this patch decouples syscon object from
syscon platform driver, and allows to create syscon objects first time
when it is required by calling of syscon_regmap_lookup_by APIs and keep
a list of such syscon objects along with syscon provider device_nodes
and regmap handles.

For non-DT based platforms, this patch keeps syscon platform driver
structure so that syscon can be probed and such non-DT based drivers
can use syscon_regmap_lookup_by_pdev API and access regmap handles.
Once all users of "syscon_regmap_lookup_by_pdev" migrated to DT based,
we can completely remove platform driver of syscon, and keep only helper
functions to get regmap handles.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Suggested-by: Tomasz Figa <redacted>
Tested-by: Vivek Gautam <redacted>
Tested-by: Javier Martinez Canillas <redacted>
Signed-off-by: Pankaj Dubey <redacted>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
---
Patch v6 and related discussions can be found here [1].

Change since v5:
 - Addressed review comments from "Heiko Stuebner".
 - Updated commit description.
 - Including Arnd's and Heiko's Reviewed-by.

Change since v5:
 - Dropping creation of dummy platform device in of_syscon_register.
 - As we are changing syscon to decouple from platform_device, creation of
   dummy platform_device does not look good option, and as suggested by Arnd,
   I made another attempt so that regmap_mmio_init API should work with NULL
   dev pointer itself. Since regmap needs to know about Syscon device node
   properties so let's parse device node of syscon in syscon itself for any
   such properties and using regmap_config parameter pass all such information
   to regmap. Other concern of crashes due to NULL dev pointer in regmap already
   addressed in separate patches of regmap. Please see [2] and [3].

Changes since v4:
 - Addressed Tomasz Figa's comments for v4.
 - Added error handing in of_syscon_register function.
 - Using devm_regmap_init_mmio instead of regmap_init_mmio.

Changes since v3:
 - Addressed Arnd's comment for v2.
 - Updated of_syscon_register for adding dev pointer in regmap_init_mmio.
 - For early users created dummy platform device.

Changes since v2:
 - Added back platform device support from syscon, with one change that
   syscon will not be probed for DT based platform.
 - Added back syscon_regmap_lookup_by_pdevname API so that non-DT base
   users of syscon will not be broken.
 - Removed unwanted change in syscon.h.
 - Modified Signed-off-by list, added Suggested-by of Tomasz Figa and
   Arnd Bergmann.
 - Added Tested-by of Vivek Gautam for testing on Exynos platform.

Changes since v1:
 - Removed of_syscon_unregister function.
 - Modified of_syscon_register function and it will be used by syscon.c
   to create syscon objects whenever required.
 - Removed platform device support from syscon.
 - Removed syscon_regmap_lookup_by_pdevname API support.
 - As there are significant changes w.r.t patchset v1, I am taking over
   author for this patchset from Tomasz Figa.

[1]: https://lkml.org/lkml/2014/9/29/99
[2]: https://lkml.org/lkml/2014/9/18/130
[3]: https://lkml.org/lkml/2014/9/27/2
 drivers/mfd/syscon.c |   96 ++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 74 insertions(+), 22 deletions(-)

You probably already have enough tags, but just in case.  ;)  On an
rk3288-based system (this patch backported to 3.14):

Tested-by: Doug Anderson <dianders@chromium.org>