[personal profile] mjg59
I've temporarily got hold of a Retina Macbook Pro. Fedora 17 boots fine on it with the following kernel arguments: nointremap drm_kms_helper.poll=0 video=eDP-1:2880x1800@45e, although you'll want an updated install image. The nointremap requirement is fixed by this patch, and the others are being tracked here. The gmux chip that controls the multiple graphics chipsets has changed - patches here. That just leaves Thunderbolt.

Thunderbolt is, to a first approximation, PCIe tunnelled over a Displayport connector. It's a high-bandwidth protocol for connecting external devices. Things are complicated by it also supporting Displayport over the same connection, which I'm sure will be hilarious when we get yet another incompatible display protocol. But anyway. I picked up a Thunderbolt ethernet adapter and started poking.

Booting with the device connected looked promising - an ethernet device appeared. Pulling it out resulted in the pcie hotplug driver noticing that it was missing and cleaning up, although the b44 ethernet driver sat around looking puzzled for a while. So far so good. Unfortunately plugging it in again didn't work, and rebooting without any Thunderbolt devices connected not only did nothing on hotplug, it also meant that the Thunderbolt controller didn't show up in lspci at all.

It turns out that there's several problems at play here. The first is the missing Thunderbolt controller. The problem here is the following code in Apple's ACPI tables:
        Method (RMCR, 0, Serialized)
            If (LNot (OSDW))
                If (LAnd (LEqual (\_SB.PCI0.PEG1.UPSB.DSB1.UPS0.AVND, 0xFFFF), 
                    LEqual (\_SB.PCI0.PEG1.UPSB.DSB2.UPS0.AVND, 0xFFFF)))
                    Store ("RMCR: Disable Link and Power Off Cactus Ridge Chip",
                    Store (0x01, \_SB.PCI0.PEG1.LDIS)
                    Sleep (0x07D0)
                    Store (0x00, GP23)
This checks if the system claims to be Darwin (the core of OS X). If not, it checks whether two PCIe bridges have been configured. If both are still in an unconfigured state, it cuts the PCIe link to the upstream PCIe link and then sets GP23 to 0. Looking further, GP23 turns out to be a GPIO line. Setting it to 0 appears to cut power to the Thunderbolt controller. In summary, unless your OS claims to be OS X, booting without an attached Thunderbolt device will result in the chipset being powered down so hard that it's entirely invisible to the OS.

Fixing that is as easy as adding Darwin to the list of operating systems Linux claims to be. So, now I could boot without a connected device and still see the controller. Since ACPI seemed to be important here, I started looking at the other ACPI methods associated with the device. The first one that drew attention was Name(_GPE, 0x14). The _GPE method (not to be confused with the _GPE object at the top of the global ACPI namespace) is defined as providing the ACPI general purpose event associated with the device. Unfortunately, it's only defined as doing this for embedded controllers - Apple's use of it on a PCI device is massively non-standard. But, still, easy enough to handle. Intel chipsets map GPE 0x10 to 0x1f to GPIO lines 0-15, so this GPE is associated with GPIO 4. Looking through the ACPI code showed a bunch of other references to GP04, and it turns out that if the chip is powered down (via setting GP23 to 0) then plugging a device in to the port will generate a GPE. This makes sense - a powered down controller isn't going to notice hotplug events itself, so you need some kind of out of band notification mechanism. There's also another ACPI method that flips the polarity of the GPIO line so it doesn't keep generating events for the entire time a device is connected. It didn't take long to come up with a stub driver that would power down the controller if nothing was connected at boot, and then wake it up again on a hotplug event.

Unfortunately that's as far as I've got. I'd been hoping that everything beyond this was just PCIe hotplug, but it seems not. Apple's Thunderbolt driver is rather too large to just be handling ACPI events, and this document on Apple's website makes it pretty clear that there's OS involvement in events and device configuration. Booting with a device connected means that the firmware does the setup, but if you want to support hotplug then the OS needs to know how to do that - and Linux doesn't.

Getting this far involved rather a lot of irritation at Apple for conspiring to do things in a range of non-standard ways, but it turns out that the real villains of the piece are Intel. The Thunderbolt controller in the Apples is an Intel part - the 82524EF, according to Apple. Given Intel's enthusiasm for Linux and their generally high levels of engagement with the Linux development community, it's disappointing[1] to discover that this controller has been shipping for over a year with (a) no Linux driver and (b) no documentation that would let anyone write such a driver. It's not even mentioned on Intel's website. So, thanks Intel. You're awful.

Anyway. This was my attempt to spend a few days doing something more relaxing than secure boot, and all I ended up with was eczema and liver pain. Lesson learned, hardware vendors hate you even more than firmware vendors do.

[1] By which I mean grotesquely infuriating
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Matthew Garrett

About Matthew

Power management, mobile and firmware developer on Linux. Security developer at Google. Ex-biologist. @mjg59 on Twitter. Content here should not be interpreted as the opinion of my employer.

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