SPI TX Time Variability

Robotics & Edge Computing Jetson Systems Jetson AGX Orin
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For completeness’ sake, here are some instructions that worked for me with JetPack 6.2 (L4T 36.4.3):

How to Patch the SPI Driver on JetPack 6.2 (L4T 36.4.3)

Kernel Build & Patch Process

# Set up the kernel source directory
mkdir custom-kernel
cd custom-kernel

# Download the L4T BSP
wget https://developer.nvidia.com/downloads/embedded/l4t/r36_release_v4.3/release/Jetson_Linux_r36.4.3_aarch64.tbz2
tar xfv Jetson_Linux_r36.4.3_aarch64.tbz2

cd Linux_for_Tegra

# Apply your patch to the SPI driver
vim source/kernel/kernel-jammy-src/drivers/spi/spi-tegra114.c

# Download and extract the root filesystem
wget https://developer.nvidia.com/downloads/embedded/l4t/r36_release_v4.3/release/Tegra_Linux_Sample-Root-Filesystem_r36.4.3_aarch64.tbz2
tar -xvf Tegra_Linux_Sample-Root-Filesystem_R36.4.3_aarch64.tbz2 -C rootfs/

# Download and extract Ubuntu Jammy sources
wget https://developer.nvidia.com/downloads/embedded/l4t/r36_release_v4.3/sources/ubuntu_jammy-l4t_aarch64_src.tbz2
tar -xvf ubuntu_jammy-l4t_aarch64_src.tbz2 -C rootfs/

# Compile the kernel
cd source
sudo make -C kernel

# Set module install path
export INSTALL_MOD_PATH=../rootfs/

# Install kernel and modules
sudo -E make install -C kernel

cd kernel/kernel-jammy-src/
sudo make modules
sudo make modules_prepare
sudo -E make modules_install

# Update initrd
sudo nv-update-initrd

# Copy the new kernel image
cp arch/arm64/boot/Image /boot/ImageSPIPatch

# Update extlinux.conf to use the new kernel
sudo vim /boot/extlinux/extlinux.conf

# Reboot into the new kernel
sudo reboot

Results

  • The SPI write times observed when using the new patched kernel image, are in line with what was already mentioned in my previous post regarding Jetpack 5.1 (~60 µs per write instead of ~300 µs), jitter spikes still occur as shown in this figure:

    spi_latency_patch_JP6.2
    spi_latency_patch_JP6.21000×500 24.9 KB

Average: 68.91 µs, Median: 68.00 µs, Std Dev: 57.47 µs

Alternate Method: Patch Without Rebuilding the Kernel (applies to JP5.1 and JP6.2):

  • At the end of previous section, a new kernel object file for the spi-tegra driver would have been created in /lib/modules/$(uname -r)/kernel/drivers/spi/spi-tegra114.ko. It’s likely that not all steps above are required to create that file, I didn’t make a note of exactly which one does it.

  • As far as I could understand, the SPI driver is compiled as a module (m), and in that case you can simply replace the .ko file with the one created by the steps in the previous section (or download it below from the Artefacts section), without rebuilding the kernel. I confirmed that that was the case by running:

    zcat /proc/config.gz | grep CONFIG_SPI_TEGRA114

# Output should be: CONFIG_SPI_TEGRA114=m

Patch Steps (JetPack 5.1 & 6.2):

  1. On the target device, replace the existing .ko file:

     sudo cp spi-tegra114-with-patch.ko /lib/modules/$(uname -r)/kernel/drivers/spi/spi-tegra114.ko

  2. Regenerate module dependencies:

    sudo depmod -a

  3. Reboot:

    sudo reboot

Artefacts

Final Remarks:

  • This solution was reached through a combination of interpreting NVIDIA documentation, trial and error, and informed experimentation. If there’s a better way to achieve the same result—or if this approach introduces issues I haven’t yet encountered—feedback is very welcome and encouraged.
    Uploading: spi_latency_patch_JP6.2.png…