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In fine adjustement mode, which is the current default, the sub-second
increment register is the number of nanoseconds that will be added to
the clock when the accumulator overflows. At each clock cycle, the
value of the addend register is added to the accumulator.
Currently, we use 20ns = 1e09ns / 50MHz as this value whatever the
frequency of the ptp clock actually is.
The adjustment is then done on the addend register, only incrementing
every X clock cycles X being the ratio between 50MHz and ptp_clock_rate
(addend = 2^32 * 50MHz/ptp_clock_rate).
This causes the following issues :
- In case the frequency of the ptp clock is inferior or equal to 50MHz,
the addend value calculation will overflow and the default
addend value will be set to 0, causing the clock to not work at
all. (For instance, for ptp_clock_rate = 50MHz, addend = 2^32).
- The resolution of the timestamping clock is limited to 20ns while it
is not needed, thus limiting the accuracy of the timestamping to
20ns.
Fix this by setting sub-second increment to 2e09ns / ptp_clock_rate.
It will allow to reach the minimum possible frequency for
ptp_clk_ref, which is 5MHz for GMII 1000Mps Full-Duplex by setting the
sub-second-increment to a higher value. For instance, for 25MHz, it
gives ssinc = 80ns and default_addend = 2^31.
It will also allow to use a lower value for sub-second-increment, thus
improving the timestamping accuracy with frequencies higher than
100MHz, for instance, for 200MHz, ssinc = 10ns and default_addend =
2^31.
v1->v2:
- Remove modifications to the calculation of default addend, which broke
compatibility with clock frequencies for which 2000000000 / ptp_clk_freq
is not an integer.
- Modify description according to discussions.
Signed-off-by: Julien Beraud <julien.beraud@orolia.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.