[PATCH v3 2/2] mfd: axp20x-i2c: Document that this must be builtin on x86

[Hans de Goede]

On x86 the axp288 PMIC provides an ACPI OpRegion handler, which must be
available before other drivers using it are loaded, which can only be
ensured if the mfd, OpRegionr and i2c-bus drivers are built in.

Since the axp20x mfd code is used on non X86 too we cannot simply change
this into a bool, I've tried some Kconfig magic with if x86 but I could
not get this working correctly, so this commit just documents that this
should be builtin on x86, which fixes errors like these during boot:

mmc0: SDHCI controller on ACPI [80860F14:00] using ADMA
ACPI Error: No handler for Region [REGS] (ffff93543b0cc3a8) [UserDefinedRegion]
ACPI Error: Region UserDefinedRegion (ID=143) has no handler (20170119/exfldio-2
ACPI Error: Method parse/execution failed [\_SB.PCI0.I2C7.PMI5.GET] (Node ffff93
ACPI Error: Method parse/execution failed [\_SB.PCI0.SHC1._PS0] (Node ffff93543b
acpi 80860F14:02: Failed to change power state to D0

Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx>
---
Changes in v2:
-Fix Kconfig depends and selects to fix warning reported by kbuild test robot
-Improve commit msg (add example of ACPI errors this avoids)
Changes in v3:
-Since the axp20x mfd code is used on ARM too where it does not necessarily
 need to be builtin settle for simply documenting the need to have this
 builtin on x86
---
 drivers/mfd/Kconfig | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/mfd/Kconfig b/drivers/mfd/Kconfig
index 4eb044e..1717372 100644
--- a/drivers/mfd/Kconfig
+++ b/drivers/mfd/Kconfig
@@ -156,6 +156,11 @@ config MFD_AXP20X_I2C
 	  components like regulators or the PEK (Power Enable Key) under the
 	  corresponding menus.
 
+	  Note on x86 this provides an ACPI OpRegion, so this must be 'y'
+	  (builtin) and not a module, as the OpRegion must be available as
+	  soon as possible. For the same reason the i2c bus driver options
+	  I2C_DESIGNWARE_PLATFORM and I2C_DESIGNWARE_BAYTRAIL must be 'y' too.
+
 config MFD_AXP20X_RSB
 	tristate "X-Powers AXP series PMICs with RSB"
 	select MFD_AXP20X
-- 
2.9.3