[PATCH v2 0/2] Add syscon of_syscon_register_regmap api

[Peter Griffin]

Hi Lee, Arnd, Krzysztof, all,

This series adds support to syscon driver for a new of_syscon_register_regmap()
api.

Platforms such as gs101 require a special regmap to access PMU registers, which
in the existing upstream client drivers are accessed via syscon regmap. This
issue was partly solved in [1] whereby a custom regmap is created in exynos-pmu
and a new API exynos_get_pmu_regmap_by_phandle() created.

One issue with the approach in [1] is that it required client drivers to be
updated from syscon_regmap_lookup_by_phandle() to
exynos_get_pmu_regmap_by_phandle() when obtaining the regmap.

Whilst updating to exynos_get_pmu_regmap_by_phandle() was OK for exynos
specific drivers, it meant other drivers like syscon-reboot and syscon-poweroff
which span multiple SoC architectures could not be easily re-used.

In previous review feedback for USB phy and gs101 poweroff driver Krzysztof
requested [2] that we take a more generic approach that other SoCs can also
leverage.

The new of_syscon_register_regmap() api overcomes this limitation by allowing
a SoC driver like exynos-pmu to register it's SoC specific regmap with the
syscon driver. This keeps the SoC complexity out of syscon driver, and allows
client drivers to continue using syscon_regmap_lookup_by_phandle() as before.
The solution allows more code re-use and can be used by other SoC archs.

Notes on probe ordering

exynos-pmu runs at postcore_initcall, so all but one of the client drivers
(ufs phy, usb phy, watchdog) run after the regmap is created and registered.

The one exception to this is pinctrl-samsung driver which is also
postcore_initcall level. The exynos_get_pmu_regmap() and
exynos_get_pmu_regmap_by_phandle() have been temporarily left to support
-EPROBE_DEFER for pinctrl-samsung driver.

The longer term plan to solve that probe ordering issue is to enable
fw_devlink for syscon dt properties so they are correctly listed as
suppliers in /sys/class/devlink. I tested a PoC patch (see below) for
fw_devlink and that seemed to work fine. Once fw_devlink supports syscon I
believe exynos_get_pmu_regmap_by_phandle() api could be removed. The main issue
currently with fw_devlink syscon support is the wide diversity of dt property
naming currently in use. That was discussed previously here [3]

1248a1256,1257
> DEFINE_SUFFIX_PROP(syscon_phandle, "syscon-phandle", NULL)
> DEFINE_SUFFIX_PROP(pmu_syscon, "pmu-syscon", NULL)
1358a1368,1369
>     { .parse_prop = parse_syscon_phandle, },
>     { .parse_prop = parse_pmu_syscon, },


Note one previous concern from Saravana about syscon potentially probing
before exynos-pmu driver and it relying on drivers/Makefile ordering. I tested
this and even if mfd is listed before soc in drivers/Makefile exynos-pmu
always probes first due to syscon driver not setting a .of_match_table entry.

Once the syscon and exynos-pmu patchs are queued I will send patches for
watchdog and ufs phy drivers to switch back to syscon_regmap_lookup_by_phandle()

Many thanks,

Peter.

[1] https://lore.kernel.org/linux-arm-kernel/20240219204238.356942-1-peter.griffin@linaro.org/T/
[2] https://lore.kernel.org/lkml/06383015-51b2-4f4c-9fd8-e4f7ce12f44e@kernel.org/
[3] https://lore.kernel.org/all/CAGETcx-CCpaV7R0O0HpDpoX6KxQBuJiMmKdWA8nDE-5Qj2Sa7g@mail.gmail.com/

Changes since v1:
 - Collect by tags
 - Keep syscon lock held for check and adding entry (Krzysztof)
 - pass pmu_np not np to syscon_node_to_regmap() (William)

Link to v1:
 - https://lore.kernel.org/linux-arm-kernel/20240614140421.3172674-1-peter.griffin@linaro.org/

Peter Griffin (2):
  mfd: syscon: add of_syscon_register_regmap() API
  soc: samsung: exynos-pmu: update to use of_syscon_register_regmap()

 drivers/mfd/syscon.c             | 54 ++++++++++++++++++++++++++++++++
 drivers/soc/samsung/exynos-pmu.c | 38 +++++++++-------------
 include/linux/mfd/syscon.h       |  8 +++++
 3 files changed, 76 insertions(+), 24 deletions(-)

-- 
2.45.2.627.g7a2c4fd464-goog