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27 November 2023

Renesas Journey: Renesas strikes back

by Anne Macedo

Things are going pretty well right now and I was encouraged on going back to my Renesas kernel module exploration by my awesome girlfriend, @madu.jar.

What do we want to prove: that the kernel module erases the ROM even when it’s good to be loaded. We need to make it erase and redownload the ROM only if loading the module fails.

This is the plan:

  1. Setup a Yocto build for the Raspberry Pi CM4 (which has an amazing compute board with PCI express so I can easily test the usb board. 1.1. This Yocto build’s Kernel config must have xhci and xhci-renesas enabled as modules, not as built-ins 1.2. xhci-renesas and xhci must be blocklisted so I can enable these at any time 1.3. we also need bpftrace support on the Yocto build
  2. I have a brand new board using Renesas upd720201 chip, I need to extract its firmware before even starting it up
  3. Make the change to not redownload the firmware
  4. Run the bpftrace program that I made to track some xhci-pci-renesas module calls with the xhci-pci-renesas loaded but not xhci-pci. Then load xhci-pci and see what happens.
  5. Redump the eeprom
  6. Undo the change
  7. Run step 3 again
  8. Redump the eeprom

Yocto for raspberry pi - a story in commits

This bricked the board :c - devtool deploy-target linux-raspberrypi root@raspberrypi4-64 -s

Sometimes my board just bricks and nothing shows up. This enables more log from boot [5]

I decided to add U-boot as well to the board, just in case I brick it again. It did brick again :c

For some reason bpftrace was built with a different clang?

root@raspberrypi4-64:~# find / -name "**"
root@raspberrypi4-64:~# ln -s /usr/lib/ /usr/lib/

This solved the issue I guess.

Trying to boot the board with the PCIe board in it made u-boot get stuck on “Starting Kernel”.

Had to go back to the stock rpi firmware for PCIe to work.

Also added loglevel=7 to /boot/cmdline to enable debug logs and #define DEBUG on the renesas module.

Dumping the eeprom

I’ve got a board from AliExpress [2]. It has a D720201 chip from Renesas and a RC25Q4OE EEPROM.

Thankfully, this ROM is less exotic than the other one:

sudo ./flashrom --programmer ch341a_spi -r dump-newrenesas

flashrom 1.4.0-devel (git:) on Linux 6.4.4-arch1-1 (x86_64)
flashrom is free software, get the source code at

Using clock_gettime for delay loops (clk_id: 1, resolution: 1ns).
Found Eon flash chip "EN25F40" (512 kB, SPI) on ch341a_spi.
This flash part has status UNTESTED for operations: WP
The test status of this chip may have been updated in the latest development
version of flashrom. If you are running the latest development version,
please email a report to if any of the above operations
work correctly for you with this flash chip. Please include the flashrom log
file for all operations you tested (see the man page for details), and mention
which mainboard or programmer you tested in the subject line.
Thanks for your help!
Reading flash... done.

CM4 flashing

More info on [3].

For some reason I couldn’t find the device on Linux. On Mac, I used ./rpiboot -d msd

Tracing the renesas board

With everything set-up, let’s trace everything.

  1. modprobe xhci-pci-renesas
  2. bpftrace
  3. dmesg
  4. modprobe xhci-pci

And I saw this error:

[   55.671549] xhci_hcd 0000:01:00.0: failed to load firmware renesas_usb_fw.mem, fallback to ROM

Which meant that the board won’t even try to download the firmware, so let’s add a good firmware. I copied it from /lib/firmware/renesas_usb_fw.mem from my Arch machine.

Digging into code

Before we load the kernel module, we see that

setpci -v -s 01:00.0 f6.w
0000:01:00.0 @f6 = 8000

Future work


[1] Using devtool to modify recipes in Yocto [2] USB 3.1 PCI Express AliExpress [3] How to flash Raspberry Pi OS onto the Compute Module 4 eMMC with usbboot [4] bpftrace [5] Modifying the bootloader configuration

tags: renesas