doc: Move documents into subdirectories

This change creates the following directories under docs/
in order to provide a grouping for the content:

- components
- design
- getting_started
- perf
- process

In each of these directories an index.rst file is created
and this serves as an index / landing page for each of the
groups when the pages are compiled. Proper layout of the
top-level table of contents relies on this directory/index
structure.

Without this patch it is possible to build the documents
correctly with Sphinx but the output looks messy because
there is no overall hierarchy.

Change-Id: I3c9f4443ec98571a56a6edf775f2c8d74d7f429f
Signed-off-by: Paul Beesley <paul.beesley@arm.com>

1 files changed, 554 insertions, 0 deletions

diff --git a/docs/process/coding-guidelines.rst b/docs/process/coding-guidelines.rst
new file mode 100644
index 00000000..644f8288
--- /dev/null
+++ b/docs/process/coding-guidelines.rst
@@ -0,0 +1,554 @@
+Trusted Firmware-A Coding Guidelines
+====================================
+
+
+
+.. contents::
+
+The following sections contain TF coding guidelines. They are continually
+evolving and should not be considered "set in stone". Feel free to question them
+and provide feedback.
+
+Some of the guidelines may also apply to other codebases.
+
+**Note:** the existing TF codebase does not necessarily comply with all the
+below guidelines but the intent is for it to do so eventually.
+
+Checkpatch overrides
+--------------------
+
+Some checkpatch warnings in the TF codebase are deliberately ignored. These
+include:
+
+- ``**WARNING: line over 80 characters**``: Although the codebase should
+  generally conform to the 80 character limit this is overly restrictive in some
+  cases.
+
+- ``**WARNING: Use of volatile is usually wrong``: see
+  `Why the “volatile” type class should not be used`_ . Although this document
+  contains some very useful information, there are several legimate uses of the
+  volatile keyword within the TF codebase.
+
+Headers and inclusion
+---------------------
+
+Header guards
+^^^^^^^^^^^^^
+
+For a header file called "some_driver.h" the style used by the Trusted Firmware
+is:
+
+.. code:: c
+
+  #ifndef SOME_DRIVER_H
+  #define SOME_DRIVER_H
+
+  <header content>
+
+  #endif /* SOME_DRIVER_H */
+
+Include statement ordering
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+All header files that are included by a source file must use the following,
+grouped ordering. This is to improve readability (by making it easier to quickly
+read through the list of headers) and maintainability.
+
+#. *System* includes: Header files from the standard *C* library, such as
+   ``stddef.h`` and ``string.h``.
+
+#. *Project* includes: Header files under the ``include/`` directory within TF
+   are *project* includes.
+
+#. *Platform* includes: Header files relating to a single, specific platform,
+   and which are located under the ``plat/<platform_name>`` directory within TF,
+   are *platform* includes.
+
+Within each group, ``#include`` statements must be in alphabetical order,
+taking both the file and directory names into account.
+
+Groups must be separated by a single blank line for clarity.
+
+The example below illustrates the ordering rules using some contrived header
+file names; this type of name reuse should be otherwise avoided.
+
+.. code:: c
+
+  #include <string.h>
+
+  #include <a_dir/example/a_header.h>
+  #include <a_dir/example/b_header.h>
+  #include <a_dir/test/a_header.h>
+  #include <b_dir/example/a_header.h>
+
+  #include "./a_header.h"
+
+Include statement variants
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Two variants of the ``#include`` directive are acceptable in the TF codebase.
+Correct use of the two styles improves readability by suggesting the location
+of the included header and reducing ambiguity in cases where generic and
+platform-specific headers share a name.
+
+For header files that are in the same directory as the source file that is
+including them, use the ``"..."`` variant.
+
+For header files that are **not** in the same directory as the source file that
+is including them, use the ``<...>`` variant.
+
+Example (bl1_fwu.c):
+
+.. code:: c
+
+  #include <assert.h>
+  #include <errno.h>
+  #include <string.h>
+
+  #include "bl1_private.h"
+
+Platform include paths
+^^^^^^^^^^^^^^^^^^^^^^
+
+Platforms are allowed to add more include paths to be passed to the compiler.
+The ``PLAT_INCLUDES`` variable is used for this purpose. This is needed in
+particular for the file ``platform_def.h``.
+
+Example:
+
+.. code:: c
+
+  PLAT_INCLUDES  += -Iinclude/plat/myplat/include
+
+Types and typedefs
+------------------
+
+Use of built-in *C* and *libc* data types
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The TF codebase should be kept as portable as possible, especially since both
+64-bit and 32-bit platforms are supported. To help with this, the following data
+type usage guidelines should be followed:
+
+- Where possible, use the built-in *C* data types for variable storage (for
+  example, ``char``, ``int``, ``long long``, etc) instead of the standard *C99*
+  types. Most code is typically only concerned with the minimum size of the
+  data stored, which the built-in *C* types guarantee.
+
+- Avoid using the exact-size standard *C99* types in general (for example,
+  ``uint16_t``, ``uint32_t``, ``uint64_t``, etc) since they can prevent the
+  compiler from making optimizations. There are legitimate uses for them,
+  for example to represent data of a known structure. When using them in struct
+  definitions, consider how padding in the struct will work across architectures.
+  For example, extra padding may be introduced in AArch32 systems if a struct
+  member crosses a 32-bit boundary.
+
+- Use ``int`` as the default integer type - it's likely to be the fastest on all
+  systems. Also this can be assumed to be 32-bit as a consequence of the
+  `Procedure Call Standard for the Arm Architecture`_ and the `Procedure Call
+  Standard for the Arm 64-bit Architecture`_ .
+
+- Avoid use of ``short`` as this may end up being slower than ``int`` in some
+  systems. If a variable must be exactly 16-bit, use ``int16_t`` or
+  ``uint16_t``.
+
+- Avoid use of ``long``. This is guaranteed to be at least 32-bit but, given
+  that `int` is 32-bit on Arm platforms, there is no use for it. For integers of
+  at least 64-bit, use ``long long``.
+
+- Use ``char`` for storing text. Use ``uint8_t`` for storing other 8-bit data.
+
+- Use ``unsigned`` for integers that can never be negative (counts,
+  indices, sizes, etc). TF intends to comply with MISRA "essential type" coding
+  rules (10.X), where signed and unsigned types are considered different
+  essential types. Choosing the correct type will aid this. MISRA static
+  analysers will pick up any implicit signed/unsigned conversions that may lead
+  to unexpected behaviour.
+
+- For pointer types:
+
+  - If an argument in a function declaration is pointing to a known type then
+    simply use a pointer to that type (for example: ``struct my_struct *``).
+
+  - If a variable (including an argument in a function declaration) is pointing
+    to a general, memory-mapped address, an array of pointers or another
+    structure that is likely to require pointer arithmetic then use
+    ``uintptr_t``. This will reduce the amount of casting required in the code.
+    Avoid using ``unsigned long`` or ``unsigned long long`` for this purpose; it
+    may work but is less portable.
+
+  - For other pointer arguments in a function declaration, use ``void *``. This
+    includes pointers to types that are abstracted away from the known API and
+    pointers to arbitrary data. This allows the calling function to pass a
+    pointer argument to the function without any explicit casting (the cast to
+    ``void *`` is implicit). The function implementation can then do the
+    appropriate casting to a specific type.
+
+  - Use ``ptrdiff_t`` to compare the difference between 2 pointers.
+
+- Use ``size_t`` when storing the ``sizeof()`` something.
+
+- Use ``ssize_t`` when returning the ``sizeof()`` something from a function that
+  can also return an error code; the signed type allows for a negative return
+  code in case of error. This practice should be used sparingly.
+
+- Use ``u_register_t`` when it's important to store the contents of a register
+  in its native size (32-bit in AArch32 and 64-bit in AArch64). This is not a
+  standard *C99* type but is widely available in libc implementations,
+  including the FreeBSD version included with the TF codebase. Where possible,
+  cast the variable to a more appropriate type before interpreting the data. For
+  example, the following struct in ``ep_info.h`` could use this type to minimize
+  the storage required for the set of registers:
+
+.. code:: c
+
+    typedef struct aapcs64_params {
+            u_register_t arg0;
+            u_register_t arg1;
+            u_register_t arg2;
+            u_register_t arg3;
+            u_register_t arg4;
+            u_register_t arg5;
+            u_register_t arg6;
+            u_register_t arg7;
+    } aapcs64_params_t;
+
+If some code wants to operate on ``arg0`` and knows that it represents a 32-bit
+unsigned integer on all systems, cast it to ``unsigned int``.
+
+These guidelines should be updated if additional types are needed.
+
+Avoid anonymous typedefs of structs/enums in headers
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+For example, the following definition:
+
+.. code:: c
+
+  typedef struct {
+          int arg1;
+          int arg2;
+  } my_struct_t;
+
+
+is better written as:
+
+.. code:: c
+
+  struct my_struct {
+          int arg1;
+          int arg2;
+  };
+
+This allows function declarations in other header files that depend on the
+struct/enum to forward declare the struct/enum instead of including the
+entire header:
+
+.. code:: c
+
+  #include <my_struct.h>
+  void my_func(my_struct_t *arg);
+
+instead of:
+
+.. code:: c
+
+  struct my_struct;
+  void my_func(struct my_struct *arg);
+
+Some TF definitions use both a struct/enum name **and** a typedef name. This
+is discouraged for new definitions as it makes it difficult for TF to comply
+with MISRA rule 8.3, which states that "All declarations of an object or
+function shall use the same names and type qualifiers".
+
+The Linux coding standards also discourage new typedefs and checkpatch emits
+a warning for this.
+
+Existing typedefs will be retained for compatibility.
+
+Error handling and robustness
+-----------------------------
+
+Using CASSERT to check for compile time data errors
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Where possible, use the ``CASSERT`` macro to check the validity of data known at
+compile time instead of checking validity at runtime, to avoid unnecessary
+runtime code.
+
+For example, this can be used to check that the assembler's and compiler's views
+of the size of an array is the same.
+
+.. code:: c
+
+  #include <cassert.h>
+
+  define MY_STRUCT_SIZE 8 /* Used by assembler source files */
+
+  struct my_struct {
+      uint32_t arg1;
+      uint32_t arg2;
+  };
+
+  CASSERT(MY_STRUCT_SIZE == sizeof(struct my_struct), assert_my_struct_size_mismatch);
+
+
+If ``MY_STRUCT_SIZE`` in the above example were wrong then the compiler would
+emit an error like this:
+
+.. code:: c
+
+  my_struct.h:10:1: error: size of array ‘assert_my_struct_size_mismatch’ is negative
+
+
+Using assert() to check for programming errors
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In general, each secure world TF image (BL1, BL2, BL31 and BL32) should be
+treated as a tightly integrated package; the image builder should be aware of
+and responsible for all functionality within the image, even if code within that
+image is provided by multiple entities. This allows us to be more aggressive in
+interpreting invalid state or bad function arguments as programming errors using
+``assert()``, including arguments passed across platform porting interfaces.
+This is in contrast to code in a Linux environment, which is less tightly
+integrated and may attempt to be more defensive by passing the error back up the
+call stack.
+
+Where possible, badly written TF code should fail early using ``assert()``. This
+helps reduce the amount of untested conditional code. By default these
+statements are not compiled into release builds, although this can be overridden
+using the ``ENABLE_ASSERTIONS`` build flag.
+
+Examples:
+
+- Bad argument supplied to library function
+- Bad argument provided by platform porting function
+- Internal secure world image state is inconsistent
+
+
+Handling integration errors
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Each secure world image may be provided by a different entity (for example, a
+Trusted Boot vendor may provide the BL2 image, a TEE vendor may provide the BL32
+image and the OEM/SoC vendor may provide the other images).
+
+An image may contain bugs that are only visible when the images are integrated.
+The system integrator may not even have access to the debug variants of all the
+images in order to check if asserts are firing. For example, the release variant
+of BL1 may have already been burnt into the SoC. Therefore, TF code that detects
+an integration error should _not_ consider this a programming error, and should
+always take action, even in release builds.
+
+If an integration error is considered non-critical it should be treated as a
+recoverable error. If the error is considered critical it should be treated as
+an unexpected unrecoverable error.
+
+Handling recoverable errors
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The secure world **must not** crash when supplied with bad data from an external
+source. For example, data from the normal world or a hardware device. Similarly,
+the secure world **must not** crash if it detects a non-critical problem within
+itself or the system. It must make every effort to recover from the problem by
+emitting a ``WARN`` message, performing any necessary error handling and
+continuing.
+
+Examples:
+
+- Secure world receives SMC from normal world with bad arguments.
+- Secure world receives SMC from normal world at an unexpected time.
+- BL31 receives SMC from BL32 with bad arguments.
+- BL31 receives SMC from BL32 at unexpected time.
+- Secure world receives recoverable error from hardware device. Retrying the
+  operation may help here.
+- Non-critical secure world service is not functioning correctly.
+- BL31 SPD discovers minor configuration problem with corresponding SP.
+
+Handling unrecoverable errors
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In some cases it may not be possible for the secure world to recover from an
+error. This situation should be handled in one of the following ways:
+
+1. If the unrecoverable error is unexpected then emit an ``ERROR`` message and
+   call ``panic()``. This will end up calling the platform-specific function
+   ``plat_panic_handler()``.
+2. If the unrecoverable error is expected to occur in certain circumstances,
+   then emit an ``ERROR`` message and call the platform-specific function
+   ``plat_error_handler()``.
+
+Cases 1 and 2 are subtly different. A platform may implement ``plat_panic_handler``
+and ``plat_error_handler`` in the same way (for example, by waiting for a secure
+watchdog to time-out or by invoking an interface on the platform's power
+controller to reset the platform). However, ``plat_error_handler`` may take
+additional action for some errors (for example, it may set a flag so the
+platform resets into a different mode). Also, ``plat_panic_handler()`` may
+implement additional debug functionality (for example, invoking a hardware
+breakpoint).
+
+Examples of unexpected unrecoverable errors:
+
+- BL32 receives an unexpected SMC response from BL31 that it is unable to
+  recover from.
+- BL31 Trusted OS SPD code discovers that BL2 has not loaded the corresponding
+  Trusted OS, which is critical for platform operation.
+- Secure world discovers that a critical hardware device is an unexpected and
+  unrecoverable state.
+- Secure world receives an unexpected and unrecoverable error from a critical
+  hardware device.
+- Secure world discovers that it is running on unsupported hardware.
+
+Examples of expected unrecoverable errors:
+
+- BL1/BL2 fails to load the next image due to missing/corrupt firmware on disk.
+- BL1/BL2 fails to authenticate the next image due to an invalid certificate.
+- Secure world continuously receives recoverable errors from a hardware device
+  but is unable to proceed without a valid response.
+
+Handling critical unresponsiveness
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If the secure world is waiting for a response from an external source (for
+example, the normal world or a hardware device) which is critical for continued
+operation, it must not wait indefinitely. It must have a mechanism (for example,
+a secure watchdog) for resetting itself and/or the external source to prevent
+the system from executing in this state indefinitely.
+
+Examples:
+
+- BL1 is waiting for the normal world to raise an SMC to proceed to the next
+  stage of the secure firmware update process.
+- A Trusted OS is waiting for a response from a proxy in the normal world that
+  is critical for continued operation.
+- Secure world is waiting for a hardware response that is critical for continued
+  operation.
+
+Security considerations
+-----------------------
+
+Part of the security of a platform is handling errors correctly, as described in
+the previous section. There are several other security considerations covered in
+this section.
+
+Do not leak secrets to the normal world
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The secure world **must not** leak secrets to the normal world, for example in
+response to an SMC.
+
+Handling Denial of Service attacks
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The secure world **should never** crash or become unusable due to receiving too
+many normal world requests (a *Denial of Service* or *DoS* attack). It should
+have a mechanism for throttling or ignoring normal world requests.
+
+Performance considerations
+--------------------------
+
+Avoid printf and use logging macros
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+``debug.h`` provides logging macros (for example, ``WARN`` and ``ERROR``)
+which wrap ``tf_log`` and which allow the logging call to be compiled-out
+depending on the ``make`` command. Use these macros to avoid print statements
+being compiled unconditionally into the binary.
+
+Each logging macro has a numerical log level:
+
+.. code:: c
+
+  #define LOG_LEVEL_NONE    0
+  #define LOG_LEVEL_ERROR   10
+  #define LOG_LEVEL_NOTICE  20
+  #define LOG_LEVEL_WARNING 30
+  #define LOG_LEVEL_INFO    40
+  #define LOG_LEVEL_VERBOSE 50
+
+
+By default, all logging statements with a log level ``<= LOG_LEVEL_INFO`` will
+be compiled into debug builds and all statements with a log level
+``<= LOG_LEVEL_NOTICE`` will be compiled into release builds. This can be
+overridden from the command line or by the platform makefile (although it may be
+necessary to clean the build directory first). For example, to enable
+``VERBOSE`` logging on FVP:
+
+``make PLAT=fvp LOG_LEVEL=50 all``
+
+Use const data where possible
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+For example, the following code:
+
+.. code:: c
+
+  struct my_struct {
+          int arg1;
+          int arg2;
+  };
+
+  void init(struct my_struct *ptr);
+
+  void main(void)
+  {
+          struct my_struct x;
+          x.arg1 = 1;
+          x.arg2 = 2;
+          init(&x);
+  }
+
+is better written as:
+
+.. code:: c
+
+  struct my_struct {
+          int arg1;
+          int arg2;
+  };
+
+  void init(const struct my_struct *ptr);
+
+  void main(void)
+  {
+          const struct my_struct x = { 1, 2 };
+          init(&x);
+  }
+
+This allows the linker to put the data in a read-only data section instead of a
+writeable data section, which may result in a smaller and faster binary. Note
+that this may require dependent functions (``init()`` in the above example) to
+have ``const`` arguments, assuming they don't need to modify the data.
+
+Library and driver code
+-----------------------
+
+TF library code (under ``lib/`` and ``include/lib``) is any code that provides a
+reusable interface to other code, potentially even to code outside of TF.
+
+In some systems drivers must conform to a specific driver framework to provide
+services to the rest of the system. TF has no driver framework and the
+distinction between a driver and library is somewhat subjective.
+
+A driver (under ``drivers/`` and ``include/drivers/``) is defined as code that
+interfaces with hardware via a memory mapped interface.
+
+Some drivers (for example, the Arm CCI driver in ``include/drivers/arm/cci.h``)
+provide a general purpose API to that specific hardware. Other drivers (for
+example, the Arm PL011 console driver in ``drivers/arm/pl011/pl011_console.S``)
+provide a specific hardware implementation of a more abstract library API. In
+the latter case there may potentially be multiple drivers for the same hardware
+device.
+
+Neither libraries nor drivers should depend on platform-specific code. If they
+require platform-specific data (for example, a base address) to operate then
+they should provide an initialization function that takes the platform-specific
+data as arguments.
+
+TF common code (under ``common/`` and ``include/common/``) is code that is re-used
+by other generic (non-platform-specific) TF code. It is effectively internal
+library code.
+
+.. _`Why the “volatile” type class should not be used`: https://www.kernel.org/doc/html/latest/process/volatile-considered-harmful.html
+.. _`Procedure Call Standard for the Arm Architecture`: http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042f/IHI0042F_aapcs.pdf
+.. _`Procedure Call Standard for the Arm 64-bit Architecture`: http://infocenter.arm.com/help/topic/com.arm.doc.ihi0055b/IHI0055B_aapcs64.pdf