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Merge series from David Jander <david@protonic.nl>: These patches optimize the spi_sync call for the common case that the worker thread is idle and the queue is empty. It also opens the possibility to potentially further optimize the async path also, since it doesn't need to take into account the direct sync path anymore. As an example for the performance gain, on an i.MX8MM SoC with a SPI CAN controller attached (MCP2518FD), the time the interrupt line stays active (which corresponds roughly with the time it takes to send 3 relatively short consecutive spi_sync messages) is reduced from 98us to only 72us by this patch. A note about message ordering: This patch series should not change the behavior of message ordering when coming from the same context. This means that if a client driver issues one or more spi_async() messages immediately followed by a spi_sync() message in the same context, it can still rely on these messages being sent out in the order they were fired.
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.