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Overview

The SMI (Serial Memory Interface) serves as the memory interface that facilitates the transfer of I/Q complex samples between the CaribouLite and the RPI Broadcom SoC. This interface plays a crucial role in streaming data from the AT86RF215 IC's ADC (Analog-to-Digital Converter) to an FPGA at a maximum sample rate of 4 MSPS (Mega Samples Per Second).

Each ADC sample consists of 13 bits for the I (In-phase) component and 13 bits for the Q (Quadrature) component. These samples are then transmitted to the FPGA through the SMI interface. To accommodate the data requirements, each RF channel (CaribouLite has two channels) necessitates 4 bytes per sample, which are padded to 32 bits. This translates to a data rate of 16 MBytes/sec, equivalent to 128 MBits/sec.

In addition to the I/Q data, the Tx/Rx (Transmit/Receive) streams include flow control and configuration bits. The Microchip AT86RF215 IC, responsible for the modem functionality, features two RX I/Q outputs from its ADCs (one for each physical channel - sub-1GHz and 2.4GHz) and a single TX I/Q input directed to the DACs.

While Broadcom's Linux distribution provides the SMI interface through a device driver module embedded upon request from the Device-Tree, it lacks support for essential streaming device file APIs such as "poll," "select," and asynchronous I/O. Consequently, a custom kernel module was developed to enable uninterrupted sample streaming.

However, there are challenges associated with this approach. Developing, testing, and debugging kernel modules can be complex and time-consuming, but portability is the bigger issue. Users update their machine's kernel and each kernel update requires recompiling the kernel module using the updated kernel headers. Additionally, users with different kernel versions may need to recompile and link the kernel module locally to ensure compatibility.

This difficulty can be mitigated once the developed kernel module reaches maturity. Then, it is, hopefully, going to be integrated into the mainline linux distribution. Once this happens, the modules will be updated automatically on each system update and will not be required to recompile by the user. This also is the main issue while switching between the mainline Raspbian to DragonOS or other RPI distributions.

Currently the "smi-stream-dev" is stable but is still has certain issues to solve:

  • the usage of "single-shot" DMA request rather than "cyclic" DMA requests. As a result, depending on the systems compute load, it may miss samples and needs resynchronization. This resync process is done on the software side, but it still misses a few samples. This applies both to RX and TX, while the FPGA FIFO depth enlargement solves this problem (most of the time), this development process still needs to be visited as it will certainly make the module better and the requirements from the FPGA less constrained.
  • generalization problem: the SMI-STREAM-DEV name was given to the module for a good reason - no mentioning CaribouLite. This module should be built with no specific relation with CaribouLite and should support high speed streaming of information in the RPI (whether its a camera, display, ultrasound receiver, or SDR). Currently the module is not enough general.
  • Bugs - solving them is an iterative task. Though some of the bugs pop out frequently, tracking their source is time consuming.

Steps to Compiling

When the kernel module is not applicable to the kernel version the following is expected when starting the system:

...
      05-30 17:02:39.490  2342  2342 I CARIBOU_SMI caribou_smi_init@caribou_smi.c:493 initializing caribou_smi
      05-30 17:02:39.505  2342  2342 D CARIBOU_SMI_MODULES caribou_smi_check_modules@caribou_smi_modules.c:112 Loading smi-stream module
-->   05-30 17:02:39.506  2342  2342 E CARIBOU_SMI_MODULES caribou_smi_insert_smi_modules@caribou_smi_modules.c:71 Module insertion 'smi_stream_dev' failed
-->   05-30 17:02:39.506  2342  2342 E CARIBOU_SMI caribou_smi_init@caribou_smi.c:502 Problem reloading SMI kernel modules
-->   05-30 17:02:39.506  2342  2342 E CARIBOULITE Setup cariboulite_init_submodules@cariboulite_setup.c:286 Error setting up smi submodule
      05-30 17:02:39.506  2342  2342 D CARIBOULITE Setup cariboulite_release_submodules@cariboulite_setup.c:442 CLOSE FPGA communication
      05-30 17:02:39.506  2342  2342 I IO_UTILS_SPI io_utils_spi_remove_chip@io_utils_spi.c:475 removing an spi device with handle 0
...

This indicates that the kernel module cannot be loaded as part of your kernel and it needs to be recompiled.

The generic installation script in the root directory should solve this issue, but if we want to perform the compilation task in a more surgical manner, we need to go to the kernel module source directory software/libcariboulite/src/caribou_smi/kernel and recompile it.

  1. Make sure the system has the linux kernel headers are installed. If not, run the following: sudo apt-get -y install raspberrypi-kernel-headers module-assistant
  2. Go to the kernel module directory: cd software/libcariboulite/src/caribou_smi/kernel
  3. Create an internal build directory: mkdir build && cd build if this directory already exists, we need to delete its contents as the CMAKE generator has a search path for the kernel src directory. Once cmake is run, the the kernel-src directory path cannot be changed. The only way to re-search the directory is to delete the contents of the build subdir and regenerate makefile with CMAKE: rm -R *
  4. Generate makefile: cmake ../
  5. Build the .ko object: make
  6. Once build, the ko file is created, but an additional script is used to embed this file into the user-mode software (as a binary blob). As a result the main software needs to be recompiled just after the kernel module is compiled.

Note: This is quite a long process that can be skipped once we develop a good update / installation script that is robust. We call for ideas from the contributors to make this process faster and easier.

API

Our implemented API (user-mode) driver is located in: Caribou-SMI API Driver