[U-Boot] efi_loader: Add distro boot script for removable media

[Stephen Warren]

On 04/13/2016 12:24 PM, Alexander Graf wrote:
>
>
> On 13.04.16 19:54, Stephen Warren wrote:
>> I've spotted a couple of problems in 74522c898b35 "efi_loader: Add
>> distro boot script for removable media". These help explain something I
>> found strange in the commit description of the recently sent patch
>> "jetson-tk1: Set fdtfile environment variable"; "The 4.5.0 kernel cannot
>> cope with U-Boot's internal device tree".
>>
>>>      "load_efi_dtb="                                                   \
>>>          "load ${devtype} ${devnum}:${distro_bootpart} "           \
>>>              "${fdt_addr_r} ${prefix}${fdtfile}; "             \
>>>          "fdt addr ${fdt_addr_r}\0"                                \
>>>      \
>>
>> The "fdt addr" command shouldn't be there. That affects the DT that
>> U-Boot's internal commands operate on internally. That is entirely
>> unrelated to the the DT that is passed to the Linux kernel. Instead, the
>> EFI code should pass the DTB at ${kernel_addr_r} to the kernel, just
>> like all other boot mechanisms do.
>
> I guess you mean $fdt_addr_r?

Sorry, yes.

> I wasn't sure which one to pick back when I implemented it. "fdt addr"
> seemed like a nice fit because it gets you an explicit fdt rather than a
> "this is an address, go and see whether there happens to be a dtb loaded
> there".
>
> But I don't think it'll hurt to move it to fdt_addr_r instead.

Sounds good.

>> If by some chance U-Boot is configured by DTB and that DTB is fully
>> suitable to pass to the Linux kernel, then the board-specific code can
>> arrange for ${kernel_addr_r} to point at that same DTB, thus removing
>> the need to load one. However, that's unlikely to happen too often at
>> present; the most complete DTBs are housed in the Linux kernel source
>> tree, and DTB ABI still isn't really a thing, so in practice one mostly
>> wants to load a DTB that was built as part of the kernel being booted,
>> and hence U-Boot's DTB isn't relevant.
>
> It depends on the camp you're coming from ;). If you come from the
> traditional server camp that used to work with server class ppc in the
> past, then device tree by firmware is a pretty natural concept. Also,
> all ARM servers available today have a stable dtb ABI, because they all
> provide dtb via uEFI.
>
> In the embedded world this doesn't hold true quite as well
> unfortunately. People only realize that their device trees are
> incomplete / wrong when they write the respective drivers. So there we
> see much more churn.
>
> Over time however as a platform ages, the "embedded" style systems
> should end up with stable dtbs as well, at which point having them
> provided by U-Boot straight away is a nice compromise.

Yes, unfortunately DTB ABI or completeness isn't something that many 
downstreams think about. I'm not convinced that will change much at the 
time boards (or drivers for specific HW modules) first appear upstream. 
You're right that boards that have been around a while will tend to 
stabilize.

For embedded targets, given that we already support loading DTBs 
separately, I'm not sure I would want to change away from that. The 
system works and people are familiar with it. Switching to a different 
scheme means everyone has to adapt. Still, it might work out well on a 
case-by-case basis.