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We can currently build a multi-cpu enabled kernel that allows both ARMv4 and ARMv5 CPUs, and also supports THUMB mode in user space. However, returning to user space in this configuration with the usr_ret macro requires the use of the 'bx' instruction, which is refused by the assembler: arch/arm/kernel/entry-armv.S: Assembler messages: arch/arm/kernel/entry-armv.S:937: Error: selected processor does not support `bx lr' in ARM mode arch/arm/kernel/entry-armv.S:960: Error: selected processor does not support `bx lr' in ARM mode arch/arm/kernel/entry-armv.S:1003: Error: selected processor does not support `bx lr' in ARM mode <instantiation>:2:2: note: instruction requires: armv4t bx lr While it would be possible to handle this correctly in principle, doing so seems to not be worth it, if we can simply avoid the problem by enforcing that a kernel supporting both ARMv4 and a later CPU architecture cannot run THUMB binaries. This turned up while build-testing with clang; for some reason, gcc never triggered the problem. Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> Reviewed-by: Ard Biesheuvel <ardb@kernel.org> Signed-off-by: Arnd Bergmann <arnd@arndb.de> Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
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.