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[ Upstream commit 530e0d46c61314c59ecfdb8d3bcb87edbc0f85d3 ] The N_As value describes the time a CAN frame needs on the wire when transmitted by the CAN controller. Even very short CAN FD frames need arround 100 usecs (bitrate 1Mbit/s, data bitrate 8Mbit/s). Having N_As to be zero (the former default) leads to 'no CAN frame separation' when STmin is set to zero by the receiving node. This 'burst mode' should not be enabled by default as it could potentially dump a high number of CAN frames into the netdev queue from the soft hrtimer context. This does not affect the system stability but is just not nice and cooperative. With this N_As/frame_txtime value the 'burst mode' is disabled by default. As user space applications usually do not set the frame_txtime element of struct can_isotp_options the new in-kernel default is very likely overwritten with zero when the sockopt() CAN_ISOTP_OPTS is invoked. To make sure that a N_As value of zero is only set intentional the value '0' is now interpreted as 'do not change the current value'. When a frame_txtime of zero is required for testing purposes this CAN_ISOTP_FRAME_TXTIME_ZERO u32 value has to be set in frame_txtime. Link: https://lore.kernel.org/all/20220309120416.83514-2-socketcan@hartkopp.net Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net> Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de> Signed-off-by: Sasha Levin <sashal@kernel.org>
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.