Booting zImage with appended DTB without ATAGs support

[Pali Rohár]

On Friday 05 November 2021 11:20:01 Tom Rini wrote:
> On Fri, Nov 05, 2021 at 04:16:46PM +0100, Pali Rohár wrote:
> > So now I have a question: Do we want to support booting zImage with
> > appended DTB in U-Boot when ATAGs support is now disabled by default?
> 
> Based on your experience, yes, there are use cases for appended dtb
> booting still.  So it should be at least documented what you need to do
> where in order for that to work, and perhaps some platforms will want to
> enable it by default.

What could be implemented in U-Boot is extracting DTB from zImage+DTB
binary and boot it like if user supply separate zImage and separate DTB
files. With this approach there is no need to have ATAGs support
enabled. It would mean that kernel's code for using attached DTB would
not be used anymore as DTB would be passed to kernel separately, like
any modern boot setup.

Main issue with this approach is that if you load zImage+DTB binary from
disk or UART into memory then you loose information about total binary
size. And if you examine memory after the zImage, you cannot be sure if
data were loaded by previous command (disk read, UART transfer) or if is
just some garbage in RAM. So you can have false-positive detection that
DTB was appended.

This issue does not happen in case of booting zImage+DTB encapsulated in
uImage format, as in uImage is stored total size of that concatenated
binary. So booting via bootm should be fine. IIRC zImage+DTB-in-uImage
via bootm is used for booting new kernels on Nokia N900.

Currently affected by this issue is bootz command, which takes only
start address of the zImage binary and total size is not specified.
Command bootz takes third argument which specifies location of DTB in
memory and understand special value "-" which says to atags booting.
I'm thinking here... what about adding a new special value e.g. "+"
which would mean that DTB is attached to zImage? This could issue that
automatic detection of attached DTB into zImage is not reliable.

Any opinion?

Another approach instead of extracting DTB from zImage+DTB binary could
be to teach U-Boot to provide some simple minimalistic ATAGs and then
boot those zImage+DTB binaries like before with minimalistic ATAGs...