MIPI I3C support is now in upstream Linux

MIPI I3C specification publishedBack in August 2017, we wrote in a blog post about the first iteration of the Linux kernel subsystem we proposed to support the brand new MIPI I3C bus. Almost a year and half later, there are some really good news:

  • The Linux I3C support now has its own Git repository on kernel.org
  • The Linux I3C community has its mailing list.
  • Besides the first I3C controller driver we wrote for the Cadence I3C Master, Synopsys has contributed a second driver for their I3C master IP: i3c: master: Add driver for Synopsys DesignWare IP. This definitely helped show that there is interest in I3C beyond our contributions, and also helped validate that the subsystem was working fine for a different I3C controller.
  • The Linux I3C tree has been part of linux-next since November 6, 2018: linux-next: Tree for Nov 6.
  • The Linux I3C subsystem has received a Reviewed-by: Arnd Bergmann and later on a Acked-by: Greg Kroah-Hartman, which were two key approvals to move forward with the merging of I3C support.
  • I3C subsystem maintainer Boris Brezillon has therefore sent a pull request to get this subsystem merged in the upcoming 4.21 (or 5.0 ?) Linux kernel: [GIT PULL] i3c: Initial pull request, and this pull request has been merged by Linus Torvalds, so the I3C subsystem is now visible in Linus Git tree: drivers/i3c. The merge commit has been done on December 25, so it arrived as a very nice Christmas present!

It has been a long process, but we are proud and happy to have pioneered the support for I3C in the Linux kernel, from the design of the subsystem based on the I3C specifications all the way to its merging in the upstream Linux kernel and the creation of a small (but hopefully growing) community of developers around it.

MIPI I3C specification published, and new iteration of Linux I3C subsystem

MIPI I3C specification publishedBack in August 2017, Bootlin contributed to the Linux kernel a patch series adding support for the new MIPI I3C bus, a bus that aims at replacing busses like I2C and SPI, by offering better performance, lower power consumption, and new features like discovery, in-band interrupts and hot join.

At the time of our submission, the I3C specification was closed, but a few days ago, the MIPI Alliance announced that the I3C specification was now publicly available. This is of course very good news as it will allow a much easier and wider adoption of I3C, and it was a somewhat unexpected move since the MIPI Alliance had traditionally kept its specifications only for its members. Hopefully the I3C experience will encourage the MIPI Alliance to follow the same direction for existing or future protocols.

With this announcement from the MIPI Alliance, it was time for us to submit an updated version of our I3C support for the Linux kernel, which Bootlin engineer Boris Brezillon did on Thursday: [PATCH v2 0/7] Add the I3C subsystem. Compared to the previous version submitted in August, this new version has interesting improvements:

  • A generic infrastructure to support IBIs (in-band interrupts) was added
  • Helpers to support hot-join were added to the core I3C subsystem
  • The Cadence I3C controller driver was improved to support IBIs and hot-join
  • And of course, many of the comments received on the first iteration have been addressed

With the specification now public, we hope to receive useful comments and feedback from the Linux kernel community to improve, and hopefully in the near future, merge the support for the MIPI I3C bus.

Bootlin proposes an I3C subsystem for the Linux kernel

MIPI I3C fact sheet, from the MIPI I3C white paper
MIPI I3C fact sheet, from the MIPI I3C white paper
At the end of 2016, the MIPI consortium has finalized the first version of its I3C specification, a new communication bus that aims at replacing older busses like I2C or SPI. According to the specification, I3C gets closer to SPI data rate while requiring less pins and adding interesting mechanisms like in-band interrupts, hotplug capability or automatic discovery of devices connected on the bus. In addition, I3C provides backward compatibility with I2C: I3C and legacy I2C devices can be connected on a common bus controlled by an I3C master.

For more details about I3C, we suggest reading the MIPI I3C Whitepaper, as unfortunately MIPI has not publicly released the specifications for this protocol.

For the last few months, Bootlin engineer Boris Brezillon has been working with Cadence to develop a Linux kernel subsystem to support this new bus, as well as Cadence’s I3C master controller IP. We have now posted the first version of our patch series to the Linux kernel mailing list for review, and we already received a large number of very useful comments from the kernel community.

Bootlin is proud to be pioneering the support for this new bus in the Linux kernel, and hopes to see other developers contribute to this subsystem in the near future!