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34d76e9fa8
Through the PTP_SYS_OFFSET_EXTENDED ioctl, it is possible for userspace applications (i.e. phc2sys) to compensate for the delays incurred while reading the PHC's time. The task itself of taking the software timestamp is delegated to the SPI subsystem, through the newly introduced API in struct spi_transfer. The goal is to cross-timestamp I/O operations on the switch's PTP clock with values in the local system clock (CLOCK_REALTIME). For that we need to understand a bit of the hardware internals. The 'read PTP time' message is a 12 byte structure, first 4 bytes of which represent the SPI header, and the last 8 bytes represent the 64-bit PTP time. The switch itself starts processing the command immediately after receiving the last bit of the address, i.e. at the middle of byte 3 (last byte of header). The PTP time is shadowed to a buffer register in the switch, and retrieved atomically during the subsequent SPI frames. A similar thing goes on for the 'write PTP time' message, although in that case the switch waits until the 64-bit PTP time becomes fully available before taking any action. So the byte that needs to be software-timestamped is byte 11 (last) of the transfer. The patch creates a common (and local) sja1105_xfer implementation for the SPI I/O, and offers 3 front-ends: - sja1105_xfer_u32 and sja1105_xfer_u64: these are capable of optionally requesting a PTP timestamp - sja1105_xfer_buf: this is for large transfers (e.g. the static config buffer) and other misc data, and there is no point in giving timestamping capabilities to this. Signed-off-by: Vladimir Oltean <olteanv@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
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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.