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MCP251XFD_REG_TBC is the time base counter register. It increments once per SYS clock tick, which is 20 or 40 MHz. Observation shows that if the lowest byte (which is transferred first on the SPI bus) of that register is 0x00 or 0x80 the calculated CRC doesn't always match the transferred one. To reproduce this problem let the driver read the TBC register in a high frequency. This can be done by attaching only the mcp251xfd CAN controller to a valid terminated CAN bus and send a single CAN frame. As there are no other CAN controller on the bus, the sent CAN frame is not ACKed and the mcp251xfd repeats it. If user space enables the bus error reporting, each of the NACK errors is reported with a time stamp (which is read from the TBC register) to user space. $ ip link set can0 down $ ip link set can0 up type can bitrate 500000 berr-reporting on $ cansend can0 4FF#ff.01.00.00.00.00.00.00 This leads to several error messages per second: | mcp251xfd spi0.0 can0: CRC read error at address 0x0010 (length=4, data=00 3a 86 da, CRC=0x7753) retrying. | mcp251xfd spi0.0 can0: CRC read error at address 0x0010 (length=4, data=80 01 b4 da, CRC=0x5830) retrying. | mcp251xfd spi0.0 can0: CRC read error at address 0x0010 (length=4, data=00 e9 23 db, CRC=0xa723) retrying. | mcp251xfd spi0.0 can0: CRC read error at address 0x0010 (length=4, data=00 8a 30 db, CRC=0x4a9c) retrying. | mcp251xfd spi0.0 can0: CRC read error at address 0x0010 (length=4, data=80 f3 43 db, CRC=0x66d2) retrying. If the highest bit in the lowest byte is flipped the transferred CRC matches the calculated one. We assume for now the CRC calculation in the chip works on wrong data and the transferred data is correct. This patch implements the following workaround: - If a CRC read error on the TBC register is detected and the lowest byte is 0x00 or 0x80, the highest bit of the lowest byte is flipped and the CRC is calculated again. - If the CRC now matches, the _original_ data is passed to the reader. For now we assume transferred data was OK. Link: https://lore.kernel.org/r/20210406110617.1865592-5-mkl@pengutronix.de Cc: Manivannan Sadhasivam <mani@kernel.org> Cc: Thomas Kopp <thomas.kopp@microchip.com> Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.