Composite PCI devices in FreeBSD (mfd in Linux)

Anthony Jenkins Scoobi_doo at
Mon Dec 10 19:52:44 UTC 2018

On 12/10/18 1:26 PM, John Baldwin wrote:
> On 12/10/18 9:00 AM, Anthony Jenkins wrote:
>> 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?
> There are some existing PCI drivers that act as "virtual" busses that attach
> child devices.  For example, vga_pci.c can have drm, agp, and acpi_video
> child devices.  There are also some SMBus drivers that are also PCI-ISA
> bridges and thus create separate child devices.

Yeah I was hoping to avoid using video PCI devices as a model, as 
complex as they've gotten recently.  I'll check out its bus glue logic.

> For a virtual bus like this, you need to figure out how your child devices
> will be enumerated.  A simple way is to let child devices use an identify
> routine that looks at each parent device and decides if a child device
> for that driver makes sense.  It can then add a child device in the
> identify routine.

Really an lpss parent PCI parent device can only have the following:

  * one of {I2C, UART, SPI} controller
  * optionally an IDMA64 controller

so I was thinking a child ig4(4) device would attach to lpss iff

  * the lpss device detected an I2C controller
  * no other ig4 device is already attached

I haven't fiddled with identify() yet, will look at that tonight.

> To handle things like resources, you want to have
> bus_*_resource methods that let your child device use the normal bus_*
> functions to allocate resources.  At the simplest end you don't need to
> permit any sharing of BARs among multiple children so you can just proxy
> the requests in the "real" PCI driver.  (vga_pci.c does this)  If you need
> the BARs to be shared you have a couple of options such as just using a
> refcount on the BAR resource but letting multiple devices allocate the same
> BAR.  If you want to enforce exclusivity (once a device allocates part of
> a BAR then other children shouldn't be permitted to do so), then you will
> need a more complicated solution.

Another homework assignment for me - bus_*_resource methods.

There are 2 or 3 mutually-exclusive sub-regions in the single memory BAR:

  * 0x000 - 0x200 : I2C sub-device registers
  * 0x200 - 0x300 : lpss and I2C sub-device registers
  * 0x800 - 0x1000 : IDMA sub-device registers (optional)

The only child (ig4(4)) of a given parent lpss device would at most need 
to share access to the middle region, if at all.

> Hopefully that gives you a starting point?

Oh definitely, thanks!  If successful, and the effort to support I2C HID 
devices also comes in, it should enable a bunch of laptops to use more 
stuff like touchscreens and touchpads that are currently broken in 
FreeBSD (I'm pretty sure one of my laptop's 2 lpss devices is a 
touchscreen I2C device).


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