ARM: Add support for semihosting

Add semihosting API implementation and implement a filesystem driver
to access debugging host filesystem using it.

Tested on Freescale SabreSD board (i.MX6Q) using OpenOCD

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 57 insertions, 0 deletions

diff --git a/Documentation/filesystems/smhfs.rst b/Documentation/filesystems/smhfs.rst
new file mode 100644
index 0000000000..28de14677d
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+++ b/Documentation/filesystems/smhfs.rst
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+.. index:: smhfs (filesystem)
+
+.. _filesystems_smhfs:
+
+File I/O over ARM semihosting support
+=====================================
+
+Target Side Setup
+-----------------
+
+barebox can communicate with debug programms attached via SWD/JTAG by
+means of ARM semihosting protocol.
+
+Not all of the I/O primitives neccessary to implement a full
+filesystem are exposed in ARM semihosting API and because of that some
+aspects of filesystem funcionality are missing. Implementation does
+not have support for listing directories. This means a
+:ref:`command_ls` to a SMHFS-mounted path will show an empty
+directory. Nevertheless, the files are there.
+
+Example::
+
+  mount -t smhfs /dev/null /mnt/smhfs
+
+
+Host Side Setup
+---------------
+
+FIXME: Currently OpenOCD does not work correctly if Barebox is built
+with MMU enabled, so before using this featrue, please make sure that
+MMU is disabled in your particular configuration
+
+To make semihosting work host machine connected to the target via
+JTAG/SWD must have semihosting capable debug software running. One
+such tool would be OpenOCD. For ARM9 and ARM11 CPUs most recent
+release of OpenOCD should suffice, however for ARMv7A based devices
+patched version from here http://openocd.zylin.com/#/c/2908/ has to be
+used.
+
+The following steps are required to set up a operational semihosting
+channel:
+
+      1. In a terminal start OpenOCD and specify your particular board
+         and debug adapter used.
+
+      2. In a separate terminal connect to OpenOCD via telnet
+
+	   telnet localhost 4444
+
+      3. In resulting telnet session execute the following commands:
+
+           halt
+	   arm semihosting on
+	   resume
+
+After that is done all of the semihosting related functions should be
+ready to use.