xref: /openbmc/u-boot/doc/README.ramboot-ppc85xx (revision eba6589f)
1			RAMBOOT for MPC85xx Platforms
2			==============================
3
4RAMBOOT literally means boot from DDR. But since DDR is volatile memory some
5pre-mechanism is required to load the DDR with the bootloader binary.
6- In case of SD and SPI boot this is done by BootROM code inside the chip
7  itself.
8- In case of NAND boot FCM supports loading initial 4K code from NAND flash
9  which can initialize the DDR and get the complete bootloader copied to DDR.
10
11In addition to the above there could be some more methods to initialize the DDR
12and load it manually.
13Two of them are described below.There is also an explanation as to where these
14methods could be handy.
151. Load the RAM based bootloader onto DDR via JTAG/BDI interface. And then
16   execute the bootloader from DDR.
17   This may be handy in the following cases:
18     - In very early stage of platform bringup where other boot options are not
19       functional because of various reasons.
20     - In case the support to program the flashes on the board is not available.
21
222. Load the RAM based bootloader onto DDR using already existing bootloader on
23   the board.And then execute the bootloader from DDR.
24   Some usecases where this may be used:
25      - While developing some new feature of u-boot, for example USB driver or
26	SPI driver.
27	Suppose the board already has a working bootloader on it. And you would
28	prefer to keep it intact, at the same time want to test your bootloader.
29	In this case you can get your test bootloader binary into DDR via tftp
30	for example. Then execute the test bootloader.
31     - Suppose a platform already has a propreitery bootloader which does not
32       support for example AMP boot. In this case also RAM boot loader can be
33       utilized.
34
35   So basically when the original bootloader is required to be kept intact
36   RAM based bootloader can offer an updated bootloader on the system.
37
38Both the above Bootloaders are slight variants of SDcard or SPI Flash
39bootloader or for that matter even NAND bootloader.
40All of them define CONFIG_SYS_RAMBOOT.
41The main difference among all of them is the way the pre-environment is getting
42configured and who is doing that.
43- In case of SD card and SPI flash bootloader this is done by On Chip BootROM inside the Si itself.
44- In case of NAND boot SPL/TPL code does it with some support from Si itself.
45- In case of the pure RAM based bootloaders we have to do it by JTAG manually or already existing bootloader.
46
47How to use them:
481. Using JTAG
49   Boot up in core hold off mode or stop the core after reset using JTAG
50   interface.
51   Preconfigure DDR/L2SRAM through JTAG interface.
52	- setup DDR controller registers.
53	- setup DDR LAWs
54	- setup DDR TLB
55   Load the RAM based boot loader to the proper location in DDR/L2SRAM.
56   set up IAR (Instruction counter properly)
57   Enable the core to execute.
58
592. Using already existing bootloader.
60   get the rambased boot loader binary into DDR/L2SRAM via tftp.
61   execute the RAM based bootloader.
62      => tftp 11000000 u-boot-ram.bin
63      => go 1107f000
64
65Please note that L2SRAM can also be used instead of DDR if the SOC has
66sufficient size of L2SRAM.
67
68Necessary Code changes Required:
69=====================================
70Please note that below mentioned changes are for 85xx platforms.
71They have been tested on P1020/P2020/P1010 RDB.
72
73The main difference between the above two methods from technical perspective is
74that in 1st case SOC is just out of reset so it is in default configuration.
75(CCSRBAR is at 0xff700000).
76In the 2nd case bootloader has already re-located CCSRBAR to 0xffe00000
77
781. File name-> boards.cfg
79   There can be added specific Make options for RAMBoot. We can keep different
80   options for the two cases mentioned above.
81   for example
82   P1020RDB_JTAG_RAMBOOT and P1020RDB_GO_RAMBOOT.
83
842. platform config file
85   for example include/configs/P1_P2_RDB.h
86
87   #ifdef CONFIG_RAMBOOT
88   #define CONFIG_SDCARD
89   #endif
90
91   This will finally use the CONFIG_SYS_RAMBOOT.
92
933. Change CONFIG_SYS_CCSRBAR_DEFAULT in menuconfig accordingly.
94   In the section of the particular SOC, for example P1020, pseudo code
95
96   #if defined(CONFIG_GO)
97   #define CONFIG_SYS_CCSRBAR_DEFAULT	0xffe00000
98   #else
99   #define CONFIG_SYS_CCSRBAR_DEFAULT	0xff700000
100   #endif
101
102For JTAG  RAMBOOT this is not required because CCSRBAR is at ff700000.
103