Re: should we have a Kconfig "device group" for I2C devices?

[Philippe Mathieu-Daudé]

+Alex

On 28/1/22 15:30, Paolo Bonzini wrote:
> On 1/28/22 15:17, Peter Maydell wrote:
> > Hi; I've been looking into what is the right way to handle in Kconfig
> > an i2c device which is intended for the user to specify on the command
> > line with a -device option.
> > (It's the lsm303dlhc magnetometer, currently in code review:
> > https://patchew.org/QEMU/20210921093227.18592-1-kevin.townsend@linaro.org/  
> > )
> >
> > Currently all our i2c devices are just pulled in by "select FOO" from
> > the Kconfig stanza for a board which has that kind of sensor hardwired
> > on-board. But for at least some of them it works fine to just specify
> > them on the commandline of any board that has an i2c controller that
> > allows pluggable devices. (For instance we do that kind of commandline
> > plugging in our test suite with tests/qtest/tmp105-test.c.)
> >
> > What's the best way to structure this? For PCI we have the "device
> > group" PCI_DEVICES as documented in
> > https://qemu-project.gitlab.io/qemu/devel/kconfig.html#guidelines-for-writing-kconfig-files 
> >
> > and PCI devices say
> >      default y if PCI_DEVICES
> >      depends on PCI
> >
> > For ISA devices we seem to make them say
> >      default y
> >      depends on ISA_BUS
> >
> > I2C devices currently just say
> >      depends on I2C
> >
> > Should we have an I2C_DEVICES, which boards where there's a sensible
> > user-pluggable i2c controller can specifically select ? Or should we
> > mark the i2c devices which are sensibly user-pluggable as
> > "default y" ? Or something else ?
>
> Yes, I think it's a good idea to have I2C_DEVICES like we have 
> PCI_DEVICES.  This way we can skip them on x86 (where the SMBus 
> controller is mostly a legacy device) but include them by default on 
> AVR, embedded ARM, etc.

My first reaction was "Yes, generically each bus should have its
bus_DEVICES" switch; but then I wondered in which use case we still
need this switch.

- In the default case (--with-default-devices) if a board exposes a bus,
  we want to have all the devices compatible with the bus to be built.

- If we use --without-default-devices, then we only want the explicitly
  listed devices, and PCI_DEVICES=y here seems dubious to me.

- If we use --with-devices, this is similar to the previous case
  (--without-default-devices is a specific --with-devices case [*]).

Paolo, in what case do you see having a ${bus}_DEVICES config useful?

Thanks,

Phil.

[*] Actually I think --with-devices replaced
--with[out]-default-devices, it is more powerful / customizable;
we should only keep / maintain --with-devices and drop
--with[out]-default-devices options.