[U-Boot] [PATCH 2/2] boards: ls2080: Disable fdt copying by default

[york sun]

On 02/29/2016 09:20 PM, Bhupesh Sharma wrote:
>> From: U-Boot [mailto:u-boot-bounces at lists.denx.de] On Behalf Of Scott
>> Wood
>> Sent: Tuesday, March 01, 2016 7:13 AM
>>
>> On Tue, 2016-03-01 at 00:08 +0000, york sun wrote:
>>> Sorry for top posting. I am on outlook web access.
>>>
>>> There may be some limitation on fdt relocation. Without setting
>>> fdt_high, u -boot relocates the device tree toward the end of useable
>>> memory. I haven't got a chance to debug why it doesn't work.
>>>
>>> This patch is to disable the relocation by default. A magic number
>>> 0xa0000000 doesn't make much sense here.
>>
>> I agree, if the number is arbitrary.  But if Linux has a limitation, as
>> it does on arm32, then it should be expressed here.
>>
> 
> There are explicit requirements for placement of DTB for aarch64 linux.
> Linux versions prior to v4.2 also require that the DTB be placed within
> the 512 MB region starting at text_offset bytes below the kernel Image:
> 
> http://lxr.free-electrons.com/source/Documentation/arm64/booting.txt#L43
> 
> York - have you tested this patch against older kernels like 3.18?

Bhupesh,

Thanks for the link. It may explained why Linux doesn't boot if fdt_high is not
set properly on kernel prior 4.2.

I proposed to disable fdt copying by default. It doesn't mean we cannot will
won't use fdt_high. My point is, setting an arbitrary value doesn't make much
sense. It could be in overlap with ramdisk, or something else. Since we are
using FIT image for this board, it is easy to set correct load address for
kernel/ramdisk/dtb. Device tree can be used in place. However, it is user's
choice on how to use the memory. We can write a README as a guideline but it
makes no sense to default to 0xa0000000.

As I am working on enabling high region memory, I found it even more
inappropriate to set fdt_high to any arbitrary value. Actually, variables
including kernel_start, kernel_load, kernel_size should be removed. They don't
serve users well if the board doesn't have preloaded image to specific address,
which is almost certain on most boards. Those variables are only useful for
shipping boards from manufacture with preloaded images.

York