Thursday, June 23, 2016

What's new for Tegra in Linux v4.7

With v4.7-rc1 out[1], it's time to look at what to expect from Tegra support in Linux v4.7.


XUSB is a USB host controller that supports USB 3.0. After loading it with a proprietary firmware blob, it will expose an xHCI compliant interface and do USB 3.0 on ports that support it, as well as high-speed and full-speed where super-speed is not availabel.

The XUSB driver has been under development for a ridiculously long time. One of the reasons is that it relies on the XUSB pad controller to configure its pins as required by the board design. The XUSB pad controller is very likely one of the least-intuitive pieces of hardware I've ever encountered, and the attempts to come up with a device tree binding to describe it have been very numerous. We did finally settle on something earlier this year and after the existing code was updated for the new binding, we're finally able to support super-speed USB on Tegra124 and later.

Most of the work on this was done by Google as part of the Nyan Chromebooks and the Pixel C.

Core SoC drivers

Jon Hunter cleaned up the PMC driver and fixed a number of issues in it. He then went on to drive to conclusion another long-standing patchset that had been prototyped by Vince Hsu and me with the goal to expose the Tegra power partitions as generic power domains, which will eventually allow us to get rid of a custom Tegra API.

Some of the other core SoC drivers, such as I2C and DMA, have seen a couple of updates by Laxman Dewangan, though they've been feature-complete for the longest time, so there isn't anything really new there.


Alex Courbot has been continuing to improve support for the Tegra GPUs in the Nouveau driver. After adding support for secure boot, required by the Maxwell generation of GPUs, in v4.6, work has been ongoing in other parts of the kernel to enable the GPU on Jetson TX1. We will hopefully see that happen in the v4.8 timeframe.


Not much happened on the Tegra DRM front because I was occupied with the XUSB driver. However, a lot of code for Tegra X1 had already been merged in earlier releases, so many of the prerequisites to enable display on Jetson TX1 are in place already.

The Jetson TX1 comes with an HDMI connector that supports HDMI 2.0. The board also has a display connector that can be used to connect a DSI or eDP panel. Most of the NVIDIA engineers have a default display board equipped with a DSI panel running at a 1200x1920 resolution.


Laxman Dewangan has also added support for the Maxim MAX77620 PMIC that can be found on the P2180[2] and other Tegra X1-based boards (such as the Google Pixel C, a.k.a. Smaug). The MFD driver as well as the regulator drivers have been merged in time for v4.7, whereas a couple of others, such as the pinctrl, GPIO and RTC drivers have been merged for v4.8.

This work allows us to make good progress for the next release, because a lot of the hardware need the regulators and GPIOs exposed by these drivers for power. Patches are being worked on to enable DSI, HDMI, XUSB and GPU on the Jetson TX1.

Most 32-bit and 64-bit ARM Tegra boards now use the stdout-path property in device tree that will cause the right UART to be chosen for the debug console, so the old console kernel command-line parameter no longer needs to be passed explicitly.

Support was added for the Google Pixel C (a.k.a. Smaug), though it isn't very complete yet. It's enough to boot to a login prompt from a root filesystem on the eMMC. Further support was blocked on the MAX77620 PMIC patches, but since those have been merged, more features can now be enabled on Smaug for v4.8 and later.

  1. We're actually close to v4.7-rc5 at the time of this writing, but I had originally planned to finish this up much earlier.

  2. The P2180 is also known as the Jetson TX1 compute module. I guess it is technically called simply Jetson TX1, but everybody uses that as the short name for the Jetson TX1 Development Kit, which is a P2180 processor module on a P2597 I/O board.