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) * 1. Add your header file to U-Boot, or to include/stdpass if it is not used in * U-Boot * - * 2. Add a function below to include the header and use the struct + * 2. 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 +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 +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