arm atag retrieval problem

QIAO YANG yangqiao0505 at
Sat Jul 11 15:29:55 UTC 2015


I tried to retrieve atag command lines but I've got wired problems:
I ran a quick scan for atags, start from 0x100, and I've got an output like :

[ATAG_CORE] flags: 0, pagesize: 0, rootdev: 0
[ATAG_MEM] size: 8000000, start: 0
dma.dmachans=0x7f35 bcm2708_fb.fbwidth=656 bcm2708_fb.fbheight=416 bcm2708.boardrev=0xf bcm2708.serial=0x8247b4bb smsc95xx.macaddr=B8s
start addr:12C
 size : 2E616D64
didn't find [ATAG_NONE]

which told me that:
1. The start address of atag cmdline is 0x12C
2. The size of this tag is 0x2E616D64
The size of the tag is abnormal.
From the output, it seems that I only retrieved part of the cmdline.

I've also tried to use the mailbox interface to get the cmdline. what I've got is exactly the same part of string without error reponse. Even if I tried to print the following characters, they are all empty.
I think that the atags area maybe rewritten in some way but I didn't managed to figure out. What bothers me most is the length of the tag, because the tag id is correct, part of the value is readable, there's no reason that the data between them is incorrect. The atag area should be written by videocore, who load the cmdline.txt where we can add additional lines.

Besides I've also got a question that how can I get the value of r2 that passed by bootloader to kernel image? It should store the value of start address of ATAG or the magic of device tree. What I've found in linux is that there is a function like init( *param), which is called when loading the kernel, and the function get the value of r2 as the parameter passed to it.  I wonder how can I get this value, though by default the atag starts from 0x100.

If it takes too long to implement the feature (atag, device tree ....), I've got other ways to detect if a hdmi device is present and then disable/enable the graphic console. If I'm allowed to pass the atag/device tree implementation.

Best wishes.

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