Kconfig: Add I2C_DEVICES device group

Currently there is no way for a board model's Kconfig stanza to
say "I have an i2c bus which the user can plug an i2c device into,
build all the free-standing i2c devices". The Kconfig mechanism
for this is the "device group". Add an I2C_DEVICES group along
the same lines as the existing PCI_DEVICES. Simple free-standing
i2c devices which a user might plausibly want to be able to
plug in on the QEMU commandline should have
   default y if I2C_DEVICES
and board models which have an i2c bus that is user-accessible
should use
   imply I2C_DEVICES
to cause those pluggable devices to be built.

In this commit we mark only a fairly conservative set of i2c devices
as belonging to the I2C_DEVICES group: the simple sensors and RTCs
(not including PMBus devices or devices which need GPIO lines to be
connected).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Hao Wu <wuhaotsh@google.com>
Message-id: 20220208155911.3408455-2-peter.maydell@linaro.org

docs/devel/kconfig.rst

@@ -192,11 +192,15 @@ declares its dependencies in different ways:
   no directive and are not used in the Makefile either; they only appear
   as conditions for ``default y`` directives.
 
-  QEMU currently has two device groups, ``PCI_DEVICES`` and
-  ``TEST_DEVICES``.  PCI devices usually have a ``default y if
+  QEMU currently has three device groups, ``PCI_DEVICES``, ``I2C_DEVICES``,
+  and ``TEST_DEVICES``.  PCI devices usually have a ``default y if
   PCI_DEVICES`` directive rather than just ``default y``.  This lets
   some boards (notably s390) easily support a subset of PCI devices,
   for example only VFIO (passthrough) and virtio-pci devices.
+  ``I2C_DEVICES`` is similar to ``PCI_DEVICES``. It contains i2c devices
+  that users might reasonably want to plug in to an i2c bus on any
+  board (and not ones which are very board-specific or that need
+  to be wired up in a way that can't be done on the command line).
   ``TEST_DEVICES`` instead is used for devices that are rarely used on
   production virtual machines, but provide useful hooks to test QEMU
   or KVM.

hw/i2c/Kconfig

@@ -1,6 +1,11 @@
 config I2C
     bool
 
+config I2C_DEVICES
+    # Device group for i2c devices which can reasonably be user-plugged
+    # to any board's i2c bus
+    bool
+
 config SMBUS
     bool
     select I2C

hw/rtc/Kconfig

@@ -1,10 +1,12 @@
 config DS1338
     bool
     depends on I2C
+    default y if I2C_DEVICES
 
 config M41T80
     bool
     depends on I2C
+    default y if I2C_DEVICES
 
 config M48T59
     bool

hw/sensor/Kconfig

@@ -1,18 +1,22 @@
 config TMP105
     bool
     depends on I2C
+    default y if I2C_DEVICES
 
 config TMP421
     bool
     depends on I2C
+    default y if I2C_DEVICES
 
 config DPS310
     bool
     depends on I2C
+    default y if I2C_DEVICES
 
 config EMC141X
     bool
     depends on I2C
+    default y if I2C_DEVICES
 
 config ADM1272
     bool
@@ -25,3 +29,4 @@ config MAX34451
 config LSM303DLHC_MAG
     bool
     depends on I2C
+    default y if I2C_DEVICES