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It would be too tricky and error prone to allow DMA operations on kernel console. One of the concern is when DMA is a separate device, for example on Intel CherryTrail platforms, and might need special work around to be functional, see the commiteebb3e8d8a("ACPI / LPSS: override power state for LPSS DMA device") for more information. Another one is that kernel console is used in atomic context, e.g. when printing crucial information to the user (Oops or crash), and DMA may not serve due to power management complications including non-atomic ACPI calls but not limited to it (see above). Besides that, other concerns are described in the commit84b40e3b57("serial: 8250: omap: Disable DMA for console UART") done for OMAP UART and may be repeated here. Disable any kind of DMA operations on kernel console due to above concerns. Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com> Link: https://lore.kernel.org/r/20200217114016.49856-7-andriy.shevchenko@linux.intel.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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.