[PATCH 16/21] Documentation: Add EFI documentation

[Sascha Hauer <s.hauer@pengutronix.de>]

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
---
 Documentation/boards/efi.rst | 279 +++++++++++++++++++++++++++++++++++++++++++
 Documentation/glossary.rst   |   3 +
 2 files changed, 282 insertions(+)
 create mode 100644 Documentation/boards/efi.rst

diff --git a/Documentation/boards/efi.rst b/Documentation/boards/efi.rst
new file mode 100644
index 0000000..a8f3a09
--- /dev/null
+++ b/Documentation/boards/efi.rst
@@ -0,0 +1,279 @@
+barebox EFI support
+===================
+
+barebox can be built as an EFI application for X86 PCs. This makes
+barebox a bootloader running on PC type hardware. In EFI jargon barebox
+would be a EFI shell. Due to the barebox :ref:`bootloader_spec` support
+it can act as a replacement for gummiboot.
+
+For accessing hardware the EFI drivers and abstractions are used. barebox
+has several drivers which merely map to the underlying EFI layers. A plain
+barebox binary provides access to the screen and keyboard. The EFI System
+partition (:term:`ESP`) is available under ``boot``, additional partitions may
+be available as ``/efi*``. Networking may be available if the BIOS provides
+the necessary drivers, but most likely you'll have to download/compile
+network drivers yourself, see below.
+
+Depending on the ``CONFIG_64BIT`` option either a ia32 binary or a x86_64
+binary is built. Due to the lack of 32bit UEFI testing hardware only the
+x86_64 binary currently is tested.
+
+Building barebox for EFI
+------------------------
+
+Use the following to build barebox for EFI:
+
+.. code-block:: sh
+
+  export ARCH=efi
+  make efi_defconfig
+  make
+
+The resulting EFI image is ``barebox.efi`` (or the barebox-flash-image link).
+
+Running barebox on EFI systems
+------------------------------
+
+The simplest way to run barebox on a USB memory stick. (U)EFI only supports
+FAT filesystems, so make sure you either have a FAT16 or FAT32 filesystem on
+the memory stick. Put ``barebox.efi`` into the ``EFI/BOOT/`` directory and
+name it ``BOOTx64.EFI`` on 64bit architectures and ``BOOTIA32.EFI`` on 32bit
+architectures. Switching to USB boot in the BIOS should then be enough to
+start barebox via USB. Some BIOSes allow to specify a path to a binary to
+be executed, others have a "start UEFI shell" entry which executes
+EFI/Shellx64.efi on the :term:`ESP`. This can be a barebox binary aswell.
+
+Loading EFI applications
+========================
+
+EFI supports loading applications aswell as drivers. barebox does not differentiate
+between both. Both types can be simply executed by typing the path on the command
+line. When an application/driver returns barebox iterates over the handle database
+and will initialize all new devices.
+
+applications
+------------
+
+barebox itself and also the Linux Kernel are EFI applications. This means both
+can be directly executed. On other architectures when barebox is executed from
+another barebox it means the barebox binary will be replaced. EFI behaves
+differently, here different barebox instances will be nested, so exiting barebox
+means passing control to the calling instance. Note that currently the :ref:`command_reset`
+command will pass the control to the calling instance rather than resetting
+the CPU. This may change in the future.
+
+Although the Linux Kernel can be directly executed one should use the :ref:`command_bootm`
+command. Only the bootm command passes the Kernel commandline to the Kernel.
+
+drivers
+-------
+
+EFI is modular and drivers can be loaded during runtime. Many drivers are
+included in the BIOS already, but some have to be loaded during runtime,
+for example it's common that network drivers are not included in the BIOS.
+
+Drivers can be loaded under barebox simply by executing them:
+
+.. code-block:: sh
+
+  barebox:/ /boot/network-drivers/0001-SnpDxe.efi
+
+Should the drivers instanciate new devices these are automatically registered
+after the driver has been loaded.
+
+Simple Network Protocol (SNP)
+=============================
+
+The Simple Network Protocol provides a raw packet interface to the EFI
+network drivers. Each device which supports SNP shows up as a regular
+network device under barebox. To use SNP the BIOS must have the SNP
+protocol and the network driver installed. For getting the SNP protocol
+follow the instruction in :ref:`efi_building_edk2`. Network drivers for
+the common Intel Network devices can be found here:
+
+https://downloadcenter.intel.com/Detail_Desc.aspx?agr=Y&DwnldID=19186
+
+Once instantiated the EFI drivers take some time to bring up the link, so
+it's best to only load the network drivers when needed. This can be
+archieved with the following script to put under ``/env/network/eth0-discover``:
+
+.. code-block:: sh
+
+  #!/bin/sh
+
+  for i in /boot/network-drivers/*; do
+          $i;
+  done
+
+This script will load the drivers in ``/boot/network-drivers/`` in alphabetical
+order.
+
+**NOTE** Loading the network drivers only works when loaded in the
+correct order. First the SNP driver must be loaded and then the network device
+driver. Otherwise the drivers will load without errors, but no devices will be
+instantiated. For making the order sure the driver names can be prepended with
+a number:
+
+.. code-block:: sh
+
+  /boot/network-drivers/0001-SnpDxe.efi
+  /boot/network-drivers/0002-E6208X3.EFI
+
+It is currently not known whether this is a limitation in EFI or a bug in
+barebox.
+
+EFI File IO Interface
+=====================
+
+EFI itself has filesystem support. At least the :term:`ESP` will be mounted by the
+EFI core already. The :term:`ESP` is mounted to ``/boot`` under barebox, other devices
+are mounted to ``/efi<no>`` in no particular order.
+
+Block IO Protocol
+=================
+
+EFI provides access to block devices with the Block IO Protocol. This can
+be used to access raw block devices under barebox and also to access filesystems
+not supported by EFI. The block devices will show up as ``/dev/disk<diskno>.<partno>``
+under barebox and can be accessed like any other device:
+
+.. code-block:: sh
+
+  mount /dev/disk0.1 -text4 /mnt
+
+Care must be taken that a partition is only accessed either via the Block IO Protocol *or*
+the File IO Interface. Doing both at the same time will most likely result in data
+corruption on the partition
+
+EFI device pathes
+=================
+
+In EFI each device can be pointed to using a device path. Device pathes have multiple
+components. The toplevel component on X86 systems will be the PCI root complex, on
+other systems this can be the physical memory space. Each component will now descrive
+how to find the child component on the parent bus. Additional device path nodes can
+describe network addresses or filenames on partitions. Device pathes have a binary
+representation and a clearly defined string representation. These characteristics make
+device pathes suitable for describing boot entries. barebox could use device pathes
+to store the reference to kernels on boot media. Also device pathes could be used to
+pass a root filesystem to the Kernel.
+
+Currently device pathes are only integrated into barebox in a way that each EFI device
+has a device parameter ``devpath`` which contains its device path:
+
+.. code-block:: sh
+
+  barebox:/ echo ${handle-00000000d0012198.devpath}
+  pci_root(0)/Pci(0x1d,0x0)/Usb(0x1,0x0)/Usb(0x2,0x0)
+
+
+EFI variables
+=============
+
+EFI has support for variables which are exported via the EFI Variable Services. EFI variables
+are identified by a 64bit GUID and a name. EFI variables can have arbitrary binary values, so
+they are not compatible with barebox shell variables which can only have printable content.
+Support for these variables is not yet complete in barebox. barebox contains the efivarfs which
+has the same format as the Linux Kernels efivarfs. It can be mounted with:
+
+.. code-block:: sh
+
+  mkdir efivarfs
+  mount -tefivarfs none /efivarfs
+
+In efivarfs each variable is represented by a file named <varname>-<guid>. Access to EFI variables
+is currently readonly. Since the variables have binary content using :ref:`command_md` is often
+more suitable than :ref:`command_cat`.
+
+EFI driver model and barebox
+============================
+
+The EFI driver model is based around handles and protocols. A handle is an opaque
+cookie that represents a hardware device or a software object. Each handle can have
+multiple protocols attached to it. A protocol is a callable interface and is defined
+by a C struct containing function pointers. A protocol is identified by a 64bit GUID.
+Common examples for protocols are DEVICE_PATH, DEVICE_IO, BLOCK_IO, DISK_IO,
+FILE_SYSTEM, SIMPLE_INPUT or SIMPLE_TEXT_OUTPUT. Every handle that implements the
+DEVICE_PATH protocol is registered as device in barebox. The structure can be best
+seen in the ``devinfo`` output of such a device:
+
+.. code-block:: sh
+
+  barebox:/ devinfo handle-00000000cfaed198
+  Driver: efi-snp
+  Bus: efi
+  Protocols:
+    0: a19832b9-ac25-11d3-9a2d-0090273fc14d
+    1: 330d4706-f2a0-4e4f-a369-b66fa8d54385
+    2: e5dd1403-d622-c24e-8488-c71b17f5e802
+    3: 34d59603-1428-4429-a414-e6b3b5fd7dc1
+    4: 0e1ad94a-dcf4-11db-9705-00e08161165f
+    5: 1aced566-76ed-4218-bc81-767f1f977a89
+    6: e3161450-ad0f-11d9-9669-0800200c9a66
+    7: 09576e91-6d3f-11d2-8e39-00a0c969723b
+    8: 51dd8b21-ad8d-48e9-bc3f-24f46722c748
+  Parameters:
+    devpath: pci_root(0)/Pci(0x1c,0x3)/Pci(0x0,0x0)/Mac(e03f4914f157)
+
+The protocols section in the output shows the different protocols this
+handle implements. One of this Protocols (here the first) is the Simple
+Network Protocol GUID:
+
+.. code-block:: c
+
+  #define EFI_SIMPLE_NETWORK_PROTOCOL_GUID \
+    EFI_GUID( 0xA19832B9, 0xAC25, 0x11D3, 0x9A, 0x2D, 0x00, 0x90, 0x27, 0x3F, 0xC1, 0x4D )
+
+Matching between EFI devices and drivers is done based on the Protocol GUIDs, so
+whenever a driver GUID matches one of the GUIDs a device imeplements the drivers
+probe function is called.
+
+.. _efi_building_edk2:
+
+Building EDK2
+=============
+
+Additional drivers may be needed from the EDK2 package. For example to
+use Networking in barebox not only the network device drivers are needed,
+but also the Simple Network Protocol driver, SnpDxe.efi. This is often
+not included in the BIOS, but can be compiled from the EDK2 package.
+
+Here is only a quick walkthrough for building edk2, there are more elaborated
+HOWTOs in the net, for example on http://tianocore.sourceforge.net/wiki/Using_EDK_II_with_Native_GCC.
+
+.. code-block:: sh
+
+  git clone git://github.com/tianocore/edk2.git
+  cd edk2
+  make -C BaseTools
+  . edksetup.sh
+
+At least the following lines in ``Conf/target.txt`` should be edited::
+
+  ACTIVE_PLATFORM = MdeModulePkg/MdeModulePkg.dsc
+  TARGET_ARCH = X64
+  TOOL_CHAIN_TAG = GCC48
+  MAX_CONCURRENT_THREAD_NUMBER = 4
+
+The actual build is started with invoking ``build``. After building
+``Build/MdeModule/DEBUG_GCC48/X64/SnpDxe.efi`` should exist.
+
+**NOTE** As of this writing (July 2014) the following patch was needed to
+compile EDK2.
+
+.. code-block:: diff
+
+  diff --git a/MdeModulePkg/Universal/DebugSupportDxe/X64/AsmFuncs.S b/MdeModulePkg/Universal/DebugSupportDxe/X64/AsmFuncs.S
+  index 9783ec6..13fc06c 100644
+  --- a/MdeModulePkg/Universal/DebugSupportDxe/X64/AsmFuncs.S
+  +++ b/MdeModulePkg/Universal/DebugSupportDxe/X64/AsmFuncs.S
+  @@ -280,7 +280,7 @@ ExtraPushDone:
+
+                   mov     %ds, %rax
+                   pushq   %rax
+  -                movw    %es, %rax
+  +                mov     %es, %rax^M
+                   pushq   %rax
+                   mov     %fs, %rax
+                   pushq   %rax
+
diff --git a/Documentation/glossary.rst b/Documentation/glossary.rst
index 8bad7dc..106dce9 100644
--- a/Documentation/glossary.rst
+++ b/Documentation/glossary.rst
@@ -16,3 +16,6 @@ Glossary
 
    PBL
       Pre BootLoader image
+
+   ESP
+      EFI System Partition
-- 
2.0.0


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