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diff --git a/documentation/cbfs.txt b/documentation/cbfs.txt new file mode 100644 index 0000000000..d356b4e1a9 --- /dev/null +++ b/documentation/cbfs.txt @@ -0,0 +1,409 @@ + +Received: from www.crouse-house.com ([199.45.160.146] + for coreboot@coreboot.org; Fri, 19 Dec 2008 23:11:59 +0100 +From: Jordan Crouse <jordan@cosmicpenguin.net> + + +Greetings. I apologize for the incompleteness of what I am about to +discuss. I was planning on working on it leisurely, but my employment +circumstances changed and I've been trying to get it completed in a +hurry before I had to leave it behind. + +I've been thinking a lot about LAR lately, and ways to make it more +extensible and robust. Marc and I have been trading ideas back and +forth for a number of months, and over time a clear idea of what I +wanted to do started to take shape. + +My goal was to add small things to LAR while retaining the overall +scheme. Over time, the scheme evolved slightly, but I think you'll find +that it remains true to the original idea. Below is the beginnings of +an architecture document - I did it in text form, but if met with +aclaim, it should be wikified. This presents what I call CBFS - the +next generation LAR for next generation Coreboot. Its easier to +describe what it is by describing what changed: + +A header has been added somewhere in the bootblock similar to Carl +Daniel's scheme. In addition to the coreboot information, the header +reports the size of the ROM, the alignment of the blocks, and the offset +of the first component in the CBFS. The master header provides all +the information LAR needs plus the magic number information flashrom needs. + +Each "file" (or component, as I style them) now has a type associated +with it. The type is used by coreboot to identify the type of file that +it is loading, and it can also be used by payloads to group items in the +CBFS by type (i.e - bayou can ask for all components that are payloads). + +The header on each "file" (or component, as I like to style them) has +been simplified - We now only store the length, the type, the checksum, +and the offset to the data. The name scheme remains the same. The +addtional information, which is component specific, has been moved to +the component itself (see below). + +The components are arranged in the ROM aligned along the specified +alignment from the master header - this is to facilitate partial re-write. + +Other then that, the LAR ideas remain pretty much the same. + +The plan for moving the metadata to the components is to allow many +different kinds of components, not all of which are groked by coreboot. + However, there are three essential component types that are groked by +coreboot, and they are defined: + +stage - the stage is being parsed from the original ELF, and stored in +the ROM as a single blob of binary data. The load address, start +address, compression type and length are stored in the component sub-header. + +payload - this is essentially SELF in different clothing - same idea as +SELF, with the sub-header as above. + +optionrom - This is in flux - right now, the optionrom is stored +unadulterated and uncompressed, but that is likely to be changed. + +Following this email are two replies containing the v3 code and a new +ROM tool to implement this respectively. I told you that I was trying +to get this out before I disappear, and I'm not kidding - the code is +compile tested and not run-tested. I hope that somebody will embrace +this code and take it the rest of the way, otherwise it will die a +pretty short death. + +I realize that this will start an awesome flamewar, and I'm looking +forward to it. Thanks for listening to me over the years - and good +luck with coreboot. When you all make a million dollars, send me a few +bucks, will you? + +Jordan + +Coreboot CBFS Specification +Jordan Crouse <jordan@cosmicpenguin.net> + += Introduction = + +This document describes the coreboot CBFS specification (from here +referred to as CBFS). CBFS is a scheme for managing independent chunks +of data in a system ROM. Though not a true filesystem, the style and +concepts are similar. + + += Architecture = + +The CBFS architecture looks like the following: + +/---------------\ <-- Start of ROM +| /-----------\ | --| +| | Header | | | +| |-----------| | | +| | Name | | |-- Component +| |-----------| | | +| |Data | | | +| |.. | | | +| \-----------/ | --| +| | +| /-----------\ | +| | Header | | +| |-----------| | +| | Name | | +| |-----------| | +| |Data | | +| |.. | | +| \-----------/ | +| | +| ... | +| /-----------\ | +| | | | +| | Bootblock | | +| | --------- | | +| | Reset | | <- 0xFFFFFFF0 +| \-----------/ | +\---------------/ + + +The CBFS architecture consists of a binary associated with a physical +ROM disk referred hereafter as the ROM. A number of independent of +components, each with a header prepended on to data are located within +the ROM. The components are nominally arranged sequentially, though they +are aligned along a pre-defined boundary. + +The bootblock occupies the last 20k of the ROM. Within +the bootblock is a master header containing information about the ROM +including the size, alignment of the components, and the offset of the +start of the first CBFS component within the ROM. + += Master Header = + +The master header contains essential information about the ROM that is +used by both the CBFS implementation within coreboot at runtime as well +as host based utilities to create and manage the ROM. The master header +will be located somewhere within the bootblock (last 20k of the ROM). A +pointer to the location of the header will be located at offset +-12 from the end of the ROM. This translates to address 0xFFFFFFF4 on a +normal x86 system. The pointer will be to physical memory somewhere +between - 0xFFFFB000 and 0xFFFFFFF0. This makes it easier for coreboot +to locate the header at run time. Build time utilities will +need to read the pointer and do the appropriate math to locate the header. + +The following is the structure of the master header: + +struct cbfs_header { + unsigned int magic; + unsigned int size; + unsigned int align; + unsigned int offset; +}; + +The meaning of each member is as follows: + +'magic' is a 32 bit number that identifies the ROM as a CBFS type. The +magic +number is 0x4F524243, which is 'ORBC' in ASCII. + +'size' is the size of the ROM in bytes. Coreboot will subtract 'size' from +0xFFFFFFFF to locate the beginning of the ROM in memory. + +'align' is the number of bytes that each component is aligned to within the +ROM. This is used to make sure that each component is aligned correctly +with +regards to the erase block sizes on the ROM - allowing one to replace a +component at runtime without disturbing the others. + +'offset' is the offset of the the first CBFS component (from the start of +the ROM). This is to allow for arbitrary space to be left at the beginning +of the ROM for things like embedded controller firmware. + += Bootblock = +The bootblock is a mandatory component in the ROM. It is located in the +last +20k of the ROM space, and contains, among other things, the location of the +master header and the entry point for the loader firmware. The bootblock +does not have a component header attached to it. + += Components = + +CBFS components are placed in the ROM starting at 'offset' specified in +the master header and ending at the bootblock. Thus the total size +available +for components in the ROM is (ROM size - 20k - 'offset'). Each CBFS +component is to be aligned according to the 'align' value in the header. +Thus, if a component of size 1052 is located at offset 0 with an 'align' +value +of 1024, the next component will be located at offset 2048. + +Each CBFS component will be indexed with a unique ASCII string name of +unlimited size. + +Each CBFS component starts with a header: + +struct cbfs_file { + char magic[8]; + unsigned int len; + unsigned int type; + unsigned int checksum; + unsigned int offset; +}; + +'magic' is a magic value used to identify the header. During runtime, +coreboot will scan the ROM looking for this value. The default magic is +the string 'LARCHIVE'. + +'len' is the length of the data, not including the size of the header and +the size of the name. + +'type' is a 32 bit number indicating the type of data that is attached. +The data type is used in a number of ways, as detailed in the section +below. + +'checksum' is a 32bit checksum of the entire component, including the +header and name. + +'offset' is the start of the component data, based off the start of the +header. +The difference between the size of the header and offset is the size of the +component name. + +Immediately following the header will be the name of the component, +which will +null terminated and 16 byte aligned. The following picture shows the +structure of the header: + +/--------\ <- start +| Header | +|--------| <- sizeof(struct cbfs_file) +| Name | +|--------| <- 'offset' +| Data | +| ... | +\--------/ <- start + 'offset' + 'len' + +== Searching Alogrithm == + +To locate a specific component in the ROM, one starts at the 'offset' +specified in the CBFS master header. For this example, the offset will +be 0. + + From that offset, the code should search for the magic string on the +component, jumping 'align' bytes each time. So, assuming that 'align' is +16, the code will search for the string 'LARCHIVE' at offset 0, 16, 32, etc. +If the offset ever exceeds the allowable range for CBFS components, then no +component was found. + +Upon recognizing a component, the software then has to search for the +specific name of the component. This is accomplished by comparing the +desired name with the string on the component located at +offset + sizeof(struct cbfs_file). If the string matches, then the +component +has been located, otherwise the software should add 'offset' + 'len' to +the offset and resume the search for the magic value. + +== Data Types == + +The 'type' member of struct cbfs_file is used to identify the content +of the component data, and is used by coreboot and other +run-time entities to make decisions about how to handle the data. + +There are three component types that are essential to coreboot, and so +are defined here. + +=== Stages === + +Stages are code loaded by coreboot during the boot process. They are +essential to a successful boot. Stages are comprised of a single blob +of binary data that is to be loaded into a particular location in memory +and executed. The uncompressed header contains information about how +large the data is, and where it should be placed, and what additional memory +needs to be cleared. + +Stages are assigned a component value of 0x10. When coreboot sees this +component type, it knows that it should pass the data to a sub-function +that will process the stage. + +The following is the format of a stage component: + +/--------\ +| Header | +|--------| +| Binary | +| .. | +\--------/ + +The header is defined as: + +struct cbfs_stage { + unsigned int compression; + unsigned long long entry; + unsigned long long load; + unsigned int len; + unsigned int memlen; +}; + +'compression' is an integer defining how the data is compressed. There +are three compression types defined by this version of the standard: +none (0x0), lzma (0x1), and nrv2b (0x02), though additional types may be +added assuming that coreboot understands how to handle the scheme. + +'entry' is a 64 bit value indicating the location where the program +counter should jump following the loading of the stage. This should be +an absolute physical memory address. + +'load' is a 64 bit value indicating where the subsequent data should be +loaded. This should be an absolute physical memory address. + +'len' is the length of the compressed data in the component. + +'memlen' is the amount of memory that will be used by the component when +it is loaded. + +The component data will start immediately following the header. + +When coreboot loads a stage, it will first zero the memory from 'load' to +'memlen'. It will then decompress the component data according to the +specified scheme and place it in memory starting at 'load'. Following that, +it will jump execution to the address specified by 'entry'. +Some components are designed to execute directly from the ROM - coreboot +knows which components must do that and will act accordingly. + +=== Payloads === + +Payloads are loaded by coreboot following the boot process. + +Stages are assigned a component value of 0x20. When coreboot sees this +component type, it knows that it should pass the data to a sub-function +that will process the payload. Furthermore, other run time +applications such as 'bayou' may easily index all available payloads +on the system by searching for the payload type. + + +The following is the format of a stage component: + +/-----------\ +| Header | +| Segment 1 | +| Segment 2 | +| ... | +|-----------| +| Binary | +| .. | +\-----------/ + +The header is as follows: + +struct cbfs_payload { + struct cbfs_payload_segment segments; +} + +The header contains a number of segments corresponding to the segments +that need to be loaded for the payload. + +The following is the structure of each segment header: + +struct cbfs_payload_segment { + unsigned int type; + unsigned int compression; + unsigned int offset; + unsigned long long load_addr; + unsigned int len; + unsigned int mem_len; +}; + +'type' is the type of segment, one of the following: + +PAYLOAD_SEGMENT_CODE 0x45444F43 The segment contains executable code +PAYLOAD_SEGMENT_DATA 0x41544144 The segment contains data +PAYLOAD_SEGMENT_BSS 0x20535342 The memory speicfied by the segment + should be zeroed +PAYLOAD_SEGMENT_PARAMS 0x41524150 The segment contains information for + the payload +PAYLOAD_SEGMENT_ENTRY 0x52544E45 The segment contains the entry point + for the payload + +'compression' is the compression scheme for the segment. Each segment can +be independently compressed. There are three compression types defined by +this version of the standard: none (0x0), lzma (0x1), and nrv2b (0x02), +though additional types may be added assuming that coreboot understands +how to handle the scheme. + +'offset' is the address of the data within the component, starting from +the component header. + +'load_addr' is a 64 bit value indicating where the segment should be placed +in memory. + +'len' is a 32 bit value indicating the size of the segment within the +component. + +'mem_len' is the size of the data when it is placed into memory. + +The data will located immediately following the last segment. + +=== Option ROMS === + +The third specified component type will be Option ROMs. Option ROMS will +have component type '0x30'. They will have no additional header, the +uncompressed binary data will be located in the data portion of the +component. + +=== NULL === + +There is a 4th component type ,defined as NULL (0xFFFFFFFF). This is +the "don't care" component type. This can be used when the component +type is not necessary (such as when the name of the component is unique. +i.e. option_table). It is recommended that all components be assigned a +unique type, but NULL can be used when the type does not matter. |