Signed-off-by: Eric Nelson eric.nelson@boundarydevices.com --- V2 replaces the words "arch" and "architecture", since we're talking about processor variations within a family, not different processor architectures.
doc/README.imx6-multi-bom | 241 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 241 insertions(+) create mode 100644 doc/README.imx6-multi-bom
diff --git a/doc/README.imx6-multi-bom b/doc/README.imx6-multi-bom new file mode 100644 index 0000000..be66e0a --- /dev/null +++ b/doc/README.imx6-multi-bom @@ -0,0 +1,241 @@ +Supporting multiple "bills of material" on Freescale i.MX6 boards. + +This file describes how to support multiple variants of the iMX6 CPU +(i.MX6DQ and i.MX6DLS) in a single U-Boot image. + +Because memory configuration differs between the various CPU +models, auto-configuration of DDR is also covered. + +1. BACKGROUND +------------- +The Freescale i.MX6 processor family contains four variants which are pin +compatible. Refer to http://freescale.com/imx6series for details and +reference manuals, but the highlights from a U-Boot perspective are +as follows: + +i.MX6Q - Quad core, 64-bit DDR1066, 256K OCRAM +i.MX6D - Dual core, 64-bit DDR1066, 256K OCRAM +i.MX6DL - Dual core, 64-bit DDR800, 128K OCRAM +i.MX6S - Single core, 32-bit DDR800, 128K OCRAM + +These processors are also largely register-compatible, but not completely. +In particular, the IOMUX registers for common functions are in different +locations and have different selector values. + +The register addresses and values are consistent between the first two +processors in the list above (i.MX6DQ processors) and the second two +(i.MX6DLS for Dual-Lite/Solo). + +The i.MX6SL (Solo-Lite) processor is not pin compatible, so this document +does not describe support for that variant. + +Because of the pin-compatibility, a number of manufacturers produce +identical boards with BOM options for two or more of the processors. + +Similarly, identical boards are offered in a number of different memory +layouts, whether by partially populating the DRAM sockets or by populating +them with different densities of DDR. + +By following the conventions described in this document, a board can +support each of these options in a single boot image, and decrease the +overhead for managing images. + +Note that adding multi-BOM support will add to the size of the bootable +image and slow the boot process slightly. If size and speed are critical, +a configuration-specific build can be produced that removes this overhead. + +2. BOOT FLOW +------------ +The boot process for i.MX6 processors begins with execution of a first +level loader in the processor's internal ROM. This loader samples boot +pins and on-chip fuses to determine the source of the secondary boot +image. + +The boot image itself consists of a header (the DCD) which describes the +load address and payload (the U-Boot firmware). It also contains a set of +register/value pairs used to initialize the CPU prior to execution of +U-Boot. + +The boot image is produced in a final stage of the build process by the +imximage tool by processing a configuration (.cfg) file. + +In a single-BOM image, the DCD can include DDR memory initialization +values and the load address may be DDR directly. + +In order to support all of the processors in this family, the DCD must +contain a load address for the i.MX6's internal RAM (OCRAM) because the +DDR memory speed (at least) will be dependent on the processor variant. +Thankfully, the DCD items needed to load this binary are consistent +between all of the processors. + +For this reason, support for SPL (secondary program loader) is a requirement +in order to support multiple BOMS in the same image. The SPL image will +determine the processor variant and memory organization, configure the +IOMUX controller and DDR appropriately, and load either a full version of +U-Boot or an O/S. + +3. DDR configuration +-------------------- + +The DDR configuration data for single BOM boards is defined +completely within .cfg files in the various board directories. + +As of this writing, most boards use the structure defined in the directory +board/boundary/nitrogen6x/ which separates the pieces of DCD data +according to function, with this general form: + + #include "ddr-setup.cfg" + #include "1066mhz_4x128mx16.cfg" + #include "clocks.cfg" + +Note that only the second of these is specific to the CPU variant or +memory-layout, and the multi-BOM equivalent can simply omit that for +later initialization. + + #include "ddr-setup.cfg" + #include "clocks.cfg" + +In order to support the use of the memory configuration files by both the +SPL code and the imximage tool, the memory configuration files +(1066mhz_4x128mx16.cfg, et cetera) have been converted to use the DCD_REG +macro. + +In other words, this declaration in 1066mhz_4x128mx16.cfg + + DCD_REG(MX6_MMDC_P0_MDCFG0, 0x555A7974) + +will be turned into this by the preprocessor when used by imximage: + + DATA 4, MX6_MMDC_P0_MDCFG0, 0x555A7974 + +and this when used to generate memory configuration tables used by the +SPL: + {MX6_MMDC_P0_MDCFG0, 0x555A7974}, + +3. IOMUX declarations +--------------------- + +The declarations inside the header files + arch/arm/include/asm/arch-mx6/mx6q_pins.h +and + arch/arm/include/asm/arch-mx6/mx6dl_pins.h + +are used to configure the pads usage for a particular board design. + +As mentioned earlier, the register addresses and values are different +between the 6DQ and 6DLS processor sets, and these differences are +expressed in two header files: + +For i.MX6Q and i.MX6D: + arch/arm/include/asm/arch-mx6/mx6q_pins.h + +and for i.MX6DL and i.MX6S: + arch/arm/include/asm/arch-mx6/mx6dls_pins.h + +For example, the SD3_DAT2 pad is used for SD card data on all currently +supported i.MX6 boards. + +On i.MX6DQ, this is selected by writing a zero to the mux register at +address 0x020E02C8. On i.MX6DLS, the address is 0x020E031C. + +The header files mx6q_pins.h and mx6dls_pins consolidate the settings +through a macro providing a common name of SD3_DAT2__USDHC3_DAT2: + + MX6_PAD_DECL(SD3_DAT2__USDHC3_DAT2,...) + +By using the MX6_PAD_DECL macro, this can be expanded in one of three +ways, depending on the declarations of CONFIG_MX6x by a board file. +Valid options are: + + MX6Q - single BOM for i.MX6DQ + MX6DL - single BOM for i.MX6DL/S + MX6QDL - multi-BOM + +In the first two cases, the MX6_PAD_DECL macro will be expanded into +a declararation with the MX6_PAD_prefix: + MX6_PAD_name = IOMUX_PAD(...) + +In the last case, the MX6_PAD_DECL macro will be expanded into two sets +of declarations, with the prefix MX6Q_PAD_ for the i.MX6DQ pads and the +prefix MX6DL_PAD_ for the i.MX6DLS pads. + +This is accomplished by the header file mx6-pins.h: + #ifdef CONFIG_MX6QDL + enum { + #define MX6_PAD_DECL ... + #include "mx6q_pins.h" + + #define MX6_PAD_DECL ... + #include "mx6dl_pins.h" + }; + #elif defined(CONFIG_MX6Q) + enum { + #define MX6_PAD_DECL ... + #include "mx6q_pins.h" + }; + #elif defined(CONFIG_MX6DL) + enum { + #define MX6_PAD_DECL ... + #include "mx6dl_pins.h" + }; + #endif + +4. IOMUX usage in board files +----------------------------- + +The machinery described above is sufficient to allow a set of pad +registers to be defined for a specific BOM: + + static iomux_v3_cfg_t const mypads[] = { + MX6_PAD_x, + ... + }; + +or multiple BOMs: + static iomux_v3_cfg_t const mx6q_pads[] = { + MX6Q_PAD_x, + ... + }; + + static iomux_v3_cfg_t const mx6dl_pads[] = { + MX6DL_PAD_x, + ... + }; + +In practice, 90% of the references to pads are in these types of static +arrays, and mostly separated by usage (ethernet pads in a different array +from UART pads). + +Going forward, it is recommended that these be consolidated instead by +applicability, such that all pads that apply to both i.MX6DQ and i.MX6DLS +processor types are defined in "boardname-pads.h" with macros of this +form: MX6_PAD_DEF(PAD_REFERENCE) + +And that this file be included twice when being used in a multi-BOM build. + +e.g. + static iomux_v3_cfg_t const mx6q_nitrogen_pads[] = { + #define MX6_PAD_DEF(PAD_DEF) MX6Q_PAD_#PAD_DEF, + #include "nitrogen6x-pads.h" + }; + static iomux_v3_cfg_t const mx6dl_nitrogen_pads[] = { + #define MX6_PAD_DEF(PAD_DEF) MX6DL_PAD_#PAD_DEF, + #include "nitrogen6x-pads.h" + }}; + +Doing this allows the bulk of the pads to be defined in a single place. + +For pads that are specific to i.MX6DQ or i.MX6DLS variants, it is +recommended that they be defined directly in the board file. + +Finally, the convenience macro MX6REF(x) allows run-time selection of a +variable based on the CPU type on which the reference is made: + + imx_iomux_v3_setup_multiple_pads( + MX6REF(nitrogen_pads), + ARRAY_SIZE(MX6REF(nitrogen_pads)) + ); + +N.B. This doesn't work, since ARRAY_SIZE can't be passed this kind of +reference... +