[U-Boot] [PATCH sunxi-tools] WIP: fel: Add a command for loading U-Boot SPL binaries in eGON format

[Siarhei Siamashka]

On Thu, 5 Feb 2015 22:49:39 -0700
Simon Glass <sjg at chromium.org> wrote:

> Hi Siarhei,
> 
> On 5 February 2015 at 02:36, Siarhei Siamashka
> <siarhei.siamashka at gmail.com> wrote:
> > !!! Works only on Allwinner A20 so far and needs a bit more
> > !!! debugging. And even on A20, the transition from the SPL to
> > !!! the main U-boot binary has some glitches.
> >
> > Now it should be possible to load and execute the same U-Boot SPL,
> > as used for booting from SD cards. The only argument for this new
> > command is the name of the SPL binary file (with eGON header).
> > It can be run as:
> >
> >     fel spl u-boot-sunxi-with-spl.bin
> 
> This looks great! Does it write only the SPL portion or does it write
> U-Boot also?

Yes, it only takes care of the SPL part at the moment, but surely
this can be improved later.

The biggest inconvenience is that we need to provide the 'fel' tool
with with the magic address for the main u-boot binary. This address
is hidden in the u-boot sources:

    http://git.denx.de/?p=u-boot.git;a=blob;f=include/configs/sunxi-common.h;h=6cfd7e148900#l28

But the users need to know it in order to set in as the command
line argument for the 'fel' tool.

We could probably do a better job and store the necessary addresses
in the SPL header. Currently 'mksunxiboot' provides only a
minimalistic header with the "eGON.BT0" signature and a checksum,
because otherwise it will refuse to boot from the SD card:

    http://git.denx.de/?p=u-boot.git;a=blob;f=tools/mksunxiboot.c

But probably we can extend this header and also add the
CONFIG_SYS_TEXT_BASE value there. Together with "kernel_addr_r",
"fdt_addr_r", "scriptaddr" and "ramdisk_addr_r" values.
After this change, the 'fel' tool will be able to extract all
the necessary information from u-boot-sunxi-with-spl.bin
file without relying on the command line too much.
Moreover, these addresses are sometimes changing, and some
users are IMHO rightfully upset whenever this happens:

    http://permalink.gmane.org/gmane.comp.hardware.netbook.arm.sunxi/14919

> I don't really understand how it works but will leave that to you and Hans.
 
OK :) Anyway, now I have finally pushed a supposedly properly working
implementation to the following github branch:

    https://github.com/ssvb/sunxi-tools/commits/20150206-fel-large-spl-support

Yes, we can handle most of this stuff with Hans. But some communication
with you is still needed. In particular, we still need to decide a way
to distinguish between booting from the SD card and over USB. Your
RFC/WIP patch from
    http://lists.denx.de/pipermail/u-boot/2015-February/203858.html
was checking the value in the 'lr' register, but this is not going to
work anymore. To address this, right now I'm just replacing the
"eGON.BT0" signature in memory to a new "eGON.FEL" signature at the
address 0x0004. The idea is that this signature is critical for booting
from the SD card. If there is some other value instead, then we are
pretty sure that the SPL was not booted from the SD card. And having it
as "eGON.FEL" serves as an indication of what exactly it is.

Additionally, we might even not need to store the "lr" and "sp"
registers in save_boot_params(). If we settle on using the "eGON.FEL"
signature in memory for distinguishing between the FEL and the SD boot,
then we can also reuse the other parts of of header (for example,
the checksum) to store the return address.

As you have figured out earlier, avoiding setting VBAR and CP15
stuff is important. Right now the FEL code in the BROM has an
active interrupt handler in use (which I'm suppressing by disabling
interrupts early) and the SPL needs to keep interrupts disabled,
while trying not to mess its configuration.

-- 
Best regards,
Siarhei Siamashka