Update Linux to v5.4.2
Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/Documentation/kbuild/headers_install.rst b/Documentation/kbuild/headers_install.rst
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+=============================================
+Exporting kernel headers for use by userspace
+=============================================
+
+The "make headers_install" command exports the kernel's header files in a
+form suitable for use by userspace programs.
+
+The linux kernel's exported header files describe the API for user space
+programs attempting to use kernel services. These kernel header files are
+used by the system's C library (such as glibc or uClibc) to define available
+system calls, as well as constants and structures to be used with these
+system calls. The C library's header files include the kernel header files
+from the "linux" subdirectory. The system's libc headers are usually
+installed at the default location /usr/include and the kernel headers in
+subdirectories under that (most notably /usr/include/linux and
+/usr/include/asm).
+
+Kernel headers are backwards compatible, but not forwards compatible. This
+means that a program built against a C library using older kernel headers
+should run on a newer kernel (although it may not have access to new
+features), but a program built against newer kernel headers may not work on an
+older kernel.
+
+The "make headers_install" command can be run in the top level directory of the
+kernel source code (or using a standard out-of-tree build). It takes two
+optional arguments::
+
+ make headers_install ARCH=i386 INSTALL_HDR_PATH=/usr
+
+ARCH indicates which architecture to produce headers for, and defaults to the
+current architecture. The linux/asm directory of the exported kernel headers
+is platform-specific, to see a complete list of supported architectures use
+the command::
+
+ ls -d include/asm-* | sed 's/.*-//'
+
+INSTALL_HDR_PATH indicates where to install the headers. It defaults to
+"./usr".
+
+An 'include' directory is automatically created inside INSTALL_HDR_PATH and
+headers are installed in 'INSTALL_HDR_PATH/include'.
+
+The kernel header export infrastructure is maintained by David Woodhouse
+<dwmw2@infradead.org>.