1. Article purpose[edit source]
This article provides information needed to exchange big data between Cortex M4 and Cortex A7.
2. Introduction[edit source]
Virtual UART over RPMSG is a good solution to exchange small/medium amount of data between Cortex M4 and Cortex A7.
Some experiments have demonstrated exchange up to 7MB/s from M4 firmware to user land application.
In situation where more than 7MB/s have to be exchanged, RPMSG is not the solution; in this case transfer by DMA is a must.
DMA will decrease a lot CPU consumption due to copy of data.
But solution based on DMA requires:
- contiguous memory allocation
- physical address and size on Cortex M4
- mmap buffer to let user land application have access to it
A specific Linux driver has to be developed to take care of such constraints.
3. static architecture of big data solution[edit source]
Big data solution is composed of:
- Linux user land application
- Linux RPMSG sdb (shared data buffer) driver
- Linux RPMSG tty driver
4. behavior[edit source]
In order to demonstrate this use case, lets implement a logic analyser running on the STM32MP1 discovery board.
We will use the Arduino connector, to sample the following inputs: PE8..PE12.
A user interface will be available in a web page, running in the application.
The user interface allows to control:
- the sampling frequency
- the start / stop of the sampling
The user interface displays statistics of:
- number of compressed data received by user interface
- number of uncompressed data decompressed by user interface
- number of compressed data written by user interface in file system
At startup, the application has to:
- load Cortex M4 firmware
- open rpmsg_sdb driver
- use IOCTL function of rpmsg_sdb driver to allocate and mmap 3 buffers of 1MB in DDR memory.
- open rpmsg_tty driver
On press on START button, the application sends the sampling command to M4 firmware (including the sampling frequency), and open a "date-time.dat" binary file.
On press on STOP button, or overrun data error, or file system full error, the application sends the stop command to M4 firmware and close the "date-time.dat" binary file.
On event "buffer full" coming from Cortex M4 firmware, the application unpack data, write packed data in "date-time.dat" file, and update statistics information.
Cortex M4 firmware is responsible to:
- receive and manage DDR buffer information (address and size) in an arry
- receive sampling command and perform the sampling of PE8..PE12 in SRAM
- pack sampling data using the 3 upper bits of each data byte to implement a repetition factor
- transfer packed data to application to DDR buffer thanks to DMA
- inform application when a buffer of 1MB in DDR is full and roll to next buffer
rpmsg_sdb driver is responsible to:
- allocate / free buffers in contiguous memory (DDR)
- send physical address and size of DDR buffer
- mmap DDR buffers in order to let user land application have access to them
rpmsg_tty driver will be used to communicate (transport commands and events) between Cortex M4 firmware and user land application.
