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nxp_c45_reconstruct_ts() takes a partial hardware timestamp in @hwts, with 2 bits of the 'seconds' portion, and a full PTP time in @ts. It patches in the lower bits of @hwts into @ts, and to ensure that the reconstructed timestamp is correct, it checks whether the lower 2 bits of @hwts are not in fact higher than the lower 2 bits of @ts. This is not logically possible because, according to the calling convention, @ts was collected later in time than @hwts, but due to two's complement arithmetic it can actually happen, because the current PTP time might have wrapped around between when @hwts was collected and when @ts was, yielding the lower 2 bits of @ts smaller than those of @hwts. To correct for that situation which is expected to happen under normal conditions, the driver subtracts exactly one wraparound interval from the reconstructed timestamp, since the upper bits of that need to correspond to what the upper bits of @hwts were, not to what the upper bits of @ts were. Readers might be confused because the driver denotes the amount of bits that the partial hardware timestamp has to offer as TS_SEC_MASK (timestamp mask for seconds). But it subtracts a seemingly unrelated BIT(2), which is in fact more subtle: if the hardware timestamp provides 2 bits of partial 'seconds' timestamp, then the wraparound interval is 2^2 == BIT(2). But nonetheless, it is better to express the wraparound interval in terms of a definition we already have, so replace BIT(2) with 1 + GENMASK(1, 0) which produces the same result but is clearer. Suggested-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk> Cc: Richard Cochran <richardcochran@gmail.com> Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com> Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk> 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.