From: Simon Glass sjg@chromium.org Date: Mon, 17 Jan 2022 08:04:24 -0700
Add documentation about standard passage and update the maintainers.
Signed-off-by: Simon Glass sjg@chromium.org
Changes in v2:
- Add comments about how to pass standard passage to EFI
- Add comments about passing a bloblist to Linux
- Add detailed arch-specific information
MAINTAINERS | 10 + board/sandbox/stdpass_check.c | 10 +- doc/develop/bloblist.rst | 4 +- doc/develop/index.rst | 1 + doc/develop/std_passage.rst | 396 ++++++++++++++++++++++++++++++++++ 5 files changed, 411 insertions(+), 10 deletions(-) create mode 100644 doc/develop/std_passage.rst
diff --git a/MAINTAINERS b/MAINTAINERS index 38c68ee87d4..63723d43b63 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -1177,6 +1177,16 @@ F: common/stackprot.c F: cmd/stackprot_test.c F: test/py/tests/test_stackprotector.py
+STANDARD PASSAGE +M: Simon Glass sjg@chromium.org +F: board/sandbox/stdpass_check.c +F: cmd/bloblist.c +F: common/bloblist.c +F: doc/develop/std_passage.rst +F: include/bloblist.h +F: include/stdpass/ +F: test/bloblist.c
TARGET_BCMNS3 M: Bharat Gooty bharat.gooty@broadcom.com M: Rayagonda Kokatanur rayagonda.kokatanur@broadcom.com diff --git a/board/sandbox/stdpass_check.c b/board/sandbox/stdpass_check.c index 565124e1564..8391c7a4aed 100644 --- a/board/sandbox/stdpass_check.c +++ b/board/sandbox/stdpass_check.c @@ -8,13 +8,6 @@
#include <common.h>
-/* BLOBLISTT_U_BOOT_SPL_HANDOFF */ -#include <handoff.h> -void check_spl_handoff(void) -{
- __maybe_unused struct spl_handoff check;
-};
/*
- See also doc/develop/std_passage.rst
@@ -23,7 +16,8 @@ void check_spl_handoff(void)
- Add your header file to U-Boot, or to include/stdpass if it is not used in
- U-Boot
- Add a function below to include the header and use the struct
- Add a function below to include the header and use the struct. Please put
*/
- your function in order of tag ID (see bloblist.h)
- Template follows, see above for example
diff --git a/doc/develop/bloblist.rst b/doc/develop/bloblist.rst index 572aa65d764..e819c6dc76b 100644 --- a/doc/develop/bloblist.rst +++ b/doc/develop/bloblist.rst @@ -1,7 +1,7 @@ .. SPDX-License-Identifier: GPL-2.0+
-Blob Lists - bloblist
+Bloblist +========
Introduction
diff --git a/doc/develop/index.rst b/doc/develop/index.rst index 9592d193fca..d0aecc30059 100644 --- a/doc/develop/index.rst +++ b/doc/develop/index.rst @@ -21,6 +21,7 @@ Implementation logging makefiles menus
- std_passage uefi/index version
diff --git a/doc/develop/std_passage.rst b/doc/develop/std_passage.rst new file mode 100644 index 00000000000..461098e01e7 --- /dev/null +++ b/doc/develop/std_passage.rst @@ -0,0 +1,396 @@ +.. SPDX-License-Identifier: GPL-2.0+
+Standard Passage +================
+Introduction +------------
+It is sometimes necessary for SPL to communicate information to U-Boot proper, +such as the RAM size. This can sometimes be handled by adding the value to a +Kconfig which both SPL and U-Boot proper can use. But this does not work for +values which are detected at runtime.
+In some cases other firmware binaries are used alongside U-Boot and these may +need to pass information to U-Boot or receive information from it. In this case +there is no shared build system and it is clumsy so have to specify matching +build options across projects.
+U-Boot provides a standard way of passing information between different phases +(TPL, SPL, U-Boot). This is called `standard passage` since it creates a +standard passage through which any sort of information can flow.
+How it works +------------
+The standard passage is very simple. It is really just a way of sending a +bloblist between programs, either at a fixed address, or using registers to +indicate the location.
+A :doc:`bloblist` is a simple, contiguous data structure containing a number of +blobs. Each blob has a tag to indicate what it contains. It is designed for +simple information, like a small C struct. For more complex data, a devicetree +is preferred since it has bindings and is extensible.
+The bloblist is typically set up initially by one of the early phases of U-Boot, +such as TPL. It starts either at a fixed address or is allocated in memory using +malloc(). After that, TPL passes the location of the bloblist to SPL (using +machine register in an architecture-specific way) and SPL passes it to U-Boot +proper. It is possible to add new blobs to the bloblist at each phase. U-Boot +proper relocates the bloblist so can expand it if desired.
+Use by other projects +---------------------
+The standard passage is also intended to be used by other firmware projects, +particularly if they interface with U-Boot. It allows that project's firmware +binaries to pass information to U-Boot (if they run before U-Boot) or receive +information from U-Boot (if they run afterwards).
+These projects can copy and modify the bloblist code provided they have a +compatible license.
+Allocating tags +---------------
+Tags are defined in the `bloblist.h` header file. For the moment, the U-Boot +tree is the upstream repository for tags.
+Tags may be allocated in the following areas:
+BLOBLISTT_AREA_FIRMWARE_TOP
- A small area for tags of considerable relevance to multiple projects
+BLOBLISTT_AREA_FIRMWARE
- A larger area for tags likely to be relevant to multiple projects either now
- or in the future
+BLOBLISTT_PROJECT_AREA
- Used for specific projects that want to make sure their tags are correctly
- ignored by other binaries in the firmware flow. This area should not be
- used for tags that are used by multiple projects. Instead, use
- `BLOBLISTT_AREA_FIRMWARE`.
+BLOBLISTT_VENDOR_AREA
- Used for specific vendors that want to make sure their tags are correctly
- ignored by other binaries in the firmware flow. This area should not be
- used for tags that are used by multiple vendors. Instead, use
- `BLOBLISTT_AREA_FIRMWARE`.
+BLOBLISTT_PRIVATE_AREA
- Used for private flags. Do not send patches with these. They are for local
- or temporary use. Standard firmware binaries which see these tags in the
- bloblist typically refuse to boot.
+To add a new tag for your project, send a patch to the U-Boot project with:
- your new tag, using the next available number in the area your choose
- a header file in include/stdpass/ containing your struct definition if your
- struct is not actually used in U-Boot
- a line of code in `board/sandbox/stdpass_check.c` to use the struct (see
- that file for instructions)
+The struct definition does not need to match the code style or types names used +in the other project. For example, your project might use a type like +__UNSIGNED_INT32 which in U-Boot would be written as u32. Types should be sized +so that the struct is the same on 32- and 64-bit machines. Avoid using __packed +if possible. Instead try to order members so that it is not necessary.
+Conflicts are resolved before applying patches to mainline, so your actual tag +value may change when the patch is applied. Once your patch is accepted your tag +is allocated for all time.
+Devicetree +----------
+Devicetree has a special place in the standard passage. One of the bloblist tags +is BLOBLISTT_CONTROL_DTB which indicates that that blob contains a devicetree +intended to control U-Boot (or other binaries). This devicetree provides +hardware information and configuration in a generic way using standard bindings, +so that it is available for any project to use. The bindings are compatible with +operating systems (including Linux) so there is no need to remove them before +calling the OS.
+In cases where a binary wants to access the devicetree but does not want to +implement the bloblist feature, the offset of the devicetree within the +bloblist is provided. This avoids the need to implement bloblists just to +access the devicetree. This is a convenience feature intended for use in +degenerate cases.
+However U-Boot itself does not permit accepting a devicetree through standard +passage unless it is part of a valid bloblist. It is easy to turn on the +bloblist feature in U-Boot, so such a variation would only serve to confuse +things and encourage degeneration in other projects.
+Standard passage API +--------------------
+The protocol for passing a bloblist through the standard passage from one +binary to another is architecture-specific, meaning it works differently on +32-/64-bit ARM and x86, for example, if only because of the different register +naming.
+The bloblist is mandatory. If there is no information to pass, the bloblist must +still be provided. Following firmware stages may add blobs to the bloblist, +making use of the existing space.
+The minimum bloblist size is 256 bytes, but 4KB is recommended.
+Two registers are used to pass the bloblist and devicetree pointers. Others may +be used depending on the architecture. The protocol chosen for each architecture +should be compatible with the Linux protocol, with a way of determining whether +standard passage is used. This is done not because Linux might start using +standard passage, but to avoid reinventing the wheel.
+For 32-bit ARM:
- ========= ==============================================
- Register Contents
- ========= ==============================================
- r0 0
- r1 0xb0075701 (indicates standard passage v1)
- r2 Address of devicetree
- r3 Address of bloblist
- r4 0
- lr Return address
- ========= ==============================================
+For 64-bit ARM:
- ========= ===================================================
- Register Contents
- ========= ===================================================
- x0 Address of devicetree
- x1 0xb00757a300000001 (indicates standard passage v1)
- x2 0
- x3 Address of bloblist
- x4 0
- x30 Return address
- ========= ===================================================
+The devicetree is provided as an address but it normally included within the
^^ is?
+bloblist, meaning that searching for the BLOBLISTT_CONTROL_DTB blob with the +bloblist produces the same result as the devicetree address passed here. Having +everything in a bloblist makes it easier to manage memory, since it is all in +one contiguous block.
+Usage guidelines +----------------
+As mentioned above, blobs should contain small, simple blocks of information, +typically represented by a C structure. Using it for large or complex structures +is only permitted if these are defined by a standard byte-accurate form. +Examples include devicetree, ACPI tables, SMBIOS tables and the like. +There are also a lot of pre-existing firmware binaries which are quite complex +but qualify because it is not possible to convert them to devicetree now. Apart +from those exceptions, keep it simple!
+For complex data structures, devicetree can be used. The libfdt library has an +overhead of around 5-10KB which is small enough that most firmware binaries can +easily incorporate it. For those that must run in very constrained environments, +like U-Boot TPL, a simple blob can be used instead, as explained in the +preceding paragraph.
+Devicetree bindings must be defined so that the format of the data is well +understood. This is done through the `dt-schema`_ project, although this process +is still in its infancy.
+EFI +---
+When using EFI a different calling convention is used. For example, with 64-bit +ARM we have:
- ========= ===================================================
- Register Contents
- ========= ===================================================
- x0 handle
- x1 system table
- x30 Return address
- ========= ===================================================
+It is not possible to pass the bloblist in a register in this case. Instead, a +config table can be used. The GUID for a bloblist is +4effe9da-7728-11ec-8df6-136969a780ff
+Linux and bloblists +-------------------
+It is possible to effectively pass a bloblist to Linux, or other Operating +Systems. However it is not expected that Linux would actually implement the +bloblist data structure. How does this work?
+The bloblist sits in memory with some things in it, including a devicetree, +perhaps an SMBIOS table and a TPM log. But when U-Boot calls Linux it puts the +address/size of those individual things in the devicetree. They don't move and +are still contiguous in memory, but the bloblist around them is forgotten. Linux +doesn't know that the three separate things it is picking up are actually part +of a bloblist structure, since it doesn't care about that. Even a console log +can work the same way. There is no need to teach Linux about bloblist when it +already has a perfectly good means to accept these items (via devicetree).
+ACPI can operate in a similar way. The ACPI tables can point to things that +happen to be in a bloblist, but without any knowledge of that needed in Linux, +grub, etc. In fact the ACPI tables themselves may well be in a bloblist, as they +are in U-Boot.
+Private firmware +----------------
+Standard passage is intended to operate with open-source firmware. It may be +that closed-source firmware may use standard passage, or happen to work with it. +If so, that is a happy coincidence, but the focus here is on open-source +firmware.
+Updates +-------
+Updates and patches to this documentation are welcome. Please submit them to +the U-Boot project in the normal way.
+Design notes +------------
+This section describes some of the reasons behind the design decisions implied +by this feature.
+Why devicetree? +~~~~~~~~~~~~~~~
+Firmware is getting large and complicated, with a concomitant need for more
^^^^^^^^^^^
I had to look that word up in a dictionary...
+complex communication between binaries. We don't want to use C structs to pass +around complex data, nor invent new binary formats for everything that comes up. +The devicetree provides a useful format and is already widely used in firmware. +It supports bindings and provides validation to check for compliance, by virtue +of the Linux project and `dt-schema`_. It is easily extensible, being basically +an efficient, hierarchical, ordered dictionary.
+Some examples of how complex and annoying binary formats can become are SMBIOS +tables and Intel's Video BIOS tables. The world does not need another binary +format.
+Why not *just* devicetree? +~~~~~~~~~~~~~~~~~~~~~~~~~~
+Some early firmware binaries run in very little memory and only need to pass a +few values on to later phases. Devicetree is too heavy-weight for these cases. +For example, it is generally not possible for TPL to access a devicetree, which +is one of the motivations for the of-platdata feature.
+Why not protobuf, YAML, JSON? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+They are not as efficient and either use a lot more code or require parsing +before use. Devicetree happens to be a nice compromise.
+Why not something else? +~~~~~~~~~~~~~~~~~~~~~~~
+Possibly. Please propose it.
+Why not UUIDs? +~~~~~~~~~~~~~~
+Why use a simple integer tag instead of an 'industry-standard' UUID? Many +reasons:
+- Code should be as readable as possible. GUIDs require something like::
- EFI_GUID(0x6dadf1d1, 0xd4cc, 0x4910, \
0xbb, 0x6e, 0x82, 0xb1, 0xfd, 0x80, 0xff, 0x3d)- which is much harder to read than::
- enum {
BLOBLISTT_SPL_HANDOFF = 123,- };
+- UUIDs are more like a hash than a sequence number. Git uses them, although a
- short form of the hash is commonly shown. Why use a hash to identify
- something when we only have a small number of items?
+- We don't need to worry about collisions in open source software. We can have
- a shared repo and allocate sequence numbers from there. UUIDs come from the
- idea that everyone is working independently so people need to be able to
- allocate their own numbers and be sure that they will not conflict. This is
- needed in the PC BIOS industry where there is little shared source /
- cooperation. It is not helpful with open source.
+- UUIDs come across as just obfuscation. Does anyone know what these values
- mean? How would we look them up? Who owns which one? Is there a central
- registry?::
- EFI_GUID(0x721acf02, 0x4d77, 0x4c2a, 0xb3, 0xdc, 0x27, 0x0b, 0x7b, 0xa9, 0xe4, 0xb0)
- EFI_GUID(0xa034147d, 0x690c, 0x4154, 0x8d, 0xe6, 0xc0, 0x44, 0x64, 0x1d, 0xe9, 0x42)
- EFI_GUID(0xbbcff46c, 0xc8d3, 0x4113, 0x89, 0x85, 0xb9, 0xd4, 0xf3, 0xb3, 0xf6, 0x4e)
- EFI_GUID(0x69a79759, 0x1373, 0x4367, 0xa6, 0xc4, 0xc7, 0xf5, 0x9e, 0xfd, 0x98, 0x6e)
- EFI_GUID(0xd038747c, 0xd00c, 0x4980, 0xb3, 0x19, 0x49, 0x01, 0x99, 0xa4, 0x7d, 0x55)
- EFI_GUID(0x9c7c3aa7, 0x5332, 0x4917, 0x82, 0xb9, 0x56, 0xa5, 0xf3, 0xe6, 0x2a, 0x07)
+- It is overkill to use 16 bytes for a unique identifier in a shared project.
- It is about 10^38. Modern SoCs cannot keep that in a register and there is no
- C int type to represent it on most common hardware today! Having to check that
- adds time and code to no benefit. In early boot, space and time are
- particularly precious.
+Why contiguous? +~~~~~~~~~~~~~~~
+It is easier to allocate a single block of data than to allocate lots of little +blocks. It is easy to relocate if needed (a simple copy of a block of memory). +It can be expanded by relocating it. If we absolutely need to create a linked +list then pointers to external data can be added to a blob.
+Why bloblist? +~~~~~~~~~~~~~
+Bloblist is a simple format with an integer tag. It avoids UUIDs and meets the +requirements above. Some tweaks may be desirable to the format, but that can be +worked out in code review.
+Why pass the devicetree offset? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In cases where a binary wants to access the devicetree but does not want to +implement the bloblist feature, the offset of the devicetree within the +bloblist is provided. This avoids the need to implement bloblists just to +access the devicetree. This is a convenience feature intended for use in +degenerate cases.
+Why use registers to pass values between binaries? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It seems like the obvious solution. We could use a shared memory region with +shared configuration between projects, but that is error prone and difficult to +keep in sync.
+Why not add magic values to indicate that standard passage is used? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We could put a magic value in a register to tell the next phase that the +standard-passage information is available (in other registers). But making it +a build-time option saves at least one register and makes things a little more +deterministic at built time. If we know we can rely on it, it is easier to +use.
+.. _`dt-schema`: https://github.com/devicetree-org/dt-schema
2.34.1.703.g22d0c6ccf7-goog