Composite PCI devices in FreeBSD (mfd in Linux)

[Anthony Jenkins]

Hi all,

I'm trying to port an Intel PCI I2C controller from Linux to FreeBSD.  
Linux represents this device as an MFD (multi-function device), meaning 
it has these "sub-devices" that can be handed off to other drivers to 
actually attach devices to the system.  The Linux "super" PCI device is 
the intel-lpss-pci.c, and the "sub" device is i2c-designware-platdrv.c, 
which represents the DesignWare driver's "platform" attachment to the 
Linux system.  FreeBSD also has a DesignWare I2C controller driver, 
ig4(4), but it only has PCI and ACPI bus attachment implementations.

I have a port of the Linux intel-lpss driver to FreeBSD, but now I'm 
trying to figure out the best way to give FreeBSD's ig4(4) driver access 
to my lpss(4) device.  I'm thinking I could add an ig4_lpss.c describing 
the "attachment" of an ig4(4) to an lpss(4).  Its probe() method would 
scan the "lpss" devclass for devices, and its attach() method would 
attach itself as a child to the lpss device and "grab" the portion of 
PCI memory and the IRQ that the lpss PCI device got.

Is this the "FreeBSD Way (TM)" of handling this type of device?  If not, 
can you recommend an existing FreeBSD driver I can model my code after?
If my approach is acceptable, how do I fully describe the ig4(4) 
device's attachment to the system?  Is simply making it a child of 
lpss(4) sufficient?  It's "kind of" a PCI device (it is controlled via 
access to a PCI memory region and an IRQ), but it's a sub-device of an 
actual PCI device (lpss(4)) attached to PCI.
How would my ig4_lpss attachment get information from the lpss(4) driver 
about what it probed?

Thanks,
Anthony Jenkins

References:
  - intel-lpss.c - 
https://github.com/torvalds/linux/blob/master/drivers/mfd/intel-lpss.c
  - intel-lpss-pci.c - 
https://github.com/torvalds/linux/blob/master/drivers/mfd/intel-lpss-pci.c
  - i2c-designware-platdrv.c - 
https://github.com/torvalds/linux/blob/master/drivers/i2c/busses/i2c-designware-platdrv.c
  - "mfd: introduce a driver for LPSS devices on SPT" - 
https://lwn.net/Articles/645819/
  - FreeBSD port - https://github.com/ScoobiFreeBSD/freebsd-intel-lpss