[U-Boot] [PATCH 1/2] Pass through start.elf bootargs on Raspberry Pi

[Marco Schuster]

On Raspberry Pi, the primary bootloader start.elf uses the options in
config.txt, as well as options hidden in the firmware itself, to tell
the Linux kernel e.g. framebuffer sizes, memory regions, MAC addresses
and more.

Normally, u-boot would not be able to pass through these options to
the Linux kernel. This patch adds pass-through support via an
additional env variable "bootargs_orig" which is loaded with the
original cmdline obtained from ATAG or the FDT.

In addition, this allows the user to configure DT overlays the "Pi
way" by enabling them in config.txt, as this patch passes through the
full FDT if it has been passed one by u-boot.

Note that, as stated in the comment block, for FDT passthrough to
work, the u-boot.img must be run through a perl script by the
Raspberry Pi Foundation, which appends a trailer to the image telling
start.elf to supply a FDT instead of an ATAG struct.
---
 board/raspberrypi/rpi/rpi.c | 66 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)

diff --git a/board/raspberrypi/rpi/rpi.c b/board/raspberrypi/rpi/rpi.c
index 6451d1d..d8d0fbb 100644
--- a/board/raspberrypi/rpi/rpi.c
+++ b/board/raspberrypi/rpi/rpi.c
@@ -259,6 +259,72 @@ int misc_init_r(void)
 #ifdef CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG
        set_board_info();
 #endif
+
+       /*
+        * The RPi start.elf passes important system configuration
+        * like memory sizes and LED GPIO assignments to the kernel
+        * via kernel command line.
+        *
+        * Therefore we have to parse the passed ATAG or FDT to get the
+        * commandline and pass it through to script-land via the env
+        * variable "bootargs_orig" so it can be passed on.
+        *
+        * By default, start.elf assumes a non-DT capable kernel and
+        * passes the commandline via ATAG; this requires u-boot to
+        * load the FDT from /boot and pass it on. This works if no
+        * overlays have been passed through, but once overlays are in
+        * the mix, stuff gets complicated to do in u-boot.
+        *
+        * To force start.elf to pass a processed, ready-to-go DT,
+        * you have to use the mkknlimg tool on the u-boot image:
+        * ./mkknlimg --dtok u-boot.bin /boot/u-boot.bin
+        *
+        * mkknlimg can be obtained from
https://github.com/raspberrypi/tools/blob/master/mkimage/knlinfo
+        *
+        * User scripts can check for successful bootargs retrieval
+        * in the env variable pi_bootmode, which is either fdt, atag
+        * or unknown in case of an error.
+        *
+        * The location 0x100 is hard-coded in start.elf, and it is
+        * the same as in the default bootscripts, so you only have
+        * to omit the fatload command loading the raw FDT to get
+        * going.
+        */
+
+       void* ptr = (char*) 0x100;
+       struct tag_header* atag_ptr = ptr;
+       setenv("pi_bootmode","unknown");
+
+       if(atag_ptr->tag != ATAG_CORE) {
+               if(atag_ptr->size == be32_to_cpu(FDT_MAGIC)) {
+                       set_working_fdt_addr((ulong) ptr);
+                       int nodeoffset = fdt_path_offset(working_fdt,
"/chosen");
+                       int len;
+                       const void* nodep = fdt_getprop(working_fdt,
nodeoffset, "bootargs", &len);
+                       if(len==0) {
+                               printf("WARNING: Could not determine
bootargs from FDT!\n");
+                       } else {
+                               printf("Set bootargs_orig from FDT\n");
+                               setenv("bootargs_orig", (char*) nodep);
+                               setenv("pi_bootmode","fdt");
+                       }
+               } else {
+                       printf("Warning: start.elf did not pass ATAG
or FDT parameters\n");
+               }
+       } else {
+               while(atag_ptr->tag != ATAG_NONE) {
+                       printf("at %p, have %x (len
%x)\n",atag_ptr,atag_ptr->tag, atag_ptr->size);
+                       if(atag_ptr->tag == ATAG_CMDLINE) {
+                               char* old_cmdline = ptr + 8;
+                               printf("Set bootargs_orig from ATAG\n");
+                               setenv("bootargs_orig", old_cmdline);
+                               setenv("pi_bootmode", "atag");
+                       }
+                       ptr = ptr + (atag_ptr->size * 4);
+                       atag_ptr=ptr;
+               }
+       }
+
        return 0;
 }

--
2.6.2