board: dragonboard410c: Load U-Boot directly without LK

At the moment the U-Boot port for the DragonBoard 410c is designed
to be loaded as an Android boot image after Qualcomm's Little Kernel (LK)
bootloader. This is simple to set up but LK is redundant in this case,
since everything done by LK can be also done directly by U-Boot.

Dropping LK entirely has at least the following advantages:
  - Easier installation/board code (no need for Android boot images)
  - (Slightly) faster boot
  - Boot directly in 64-bit without a round trip to 32-bit for LK

So far this was not possible yet because of unsolved problems:

  1. Signing tool: The firmware expects a "signed" ELF image with extra
     (Qualcomm-specific) ELF headers, usually used for secure boot.
     The DragonBoard 410c does not have secure boot by default but the
     extra ELF headers are still required.

  2. PSCI bug: There seems to be a bug in the PSCI implementation
     (part of the TrustZone/tz firmware) that causes all other CPU cores
     to be started in 32-bit mode if LK is missing in the boot chain.
     This causes Linux to hang early during boot.

There is a solution for both problems now:

  1. qtestsign (https://github.com/msm8916-mainline/qtestsign)
     can be used as a "signing" tool for U-Boot and other firmware.

  2. A workaround for the "PSCI bug" is to execute the TZ syscall when
     entering U-Boot. That way PSCI is made aware of the 64-bit switch
     and starts all other CPU cores in 64-bit mode as well.

Simplify the dragonboard410c board by removing all the extra code that
is only used to build an Android boot image that can be loaded by LK.
This allows dropping the custom linker script, special image magic,
as well as most of the special build/installation instructions.

CONFIG_REMAKE_ELF is used to build a new ELF image that has both U-Boot
and the appended DTB combined. The resulting u-boot.elf can then be
passed to the "signing" tool (e.g. qtestsign).

The PSCI workaround is placed in the "boot0" hook that is enabled
with CONFIG_ENABLE_ARM_SOC_BOOT0_HOOK. The extra check for EL1 allows
compatibility with custom firmware that enters U-Boot in EL2 or EL3,
e.g. qhypstub (https://github.com/msm8916-mainline/qhypstub).

As a first step these changes apply only to DragonBoard410c.
Similar changes could likely also work for the DragonBoard 820c.

Note that removing LK wouldn't be possible that easily without a lot of
work already done three years ago by Ramon Fried. A lot of missing
initialization, pinctrl etc was already added back then even though
it was not strictly needed yet.

Cc: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>

3 files changed, 55 insertions, 0 deletions

diff --git a/doc/board/index.rst b/doc/board/index.rst
index 196b597609..a6b395238a 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -20,6 +20,7 @@ Board-specific doc
    kontron/index
    microchip/index
    openpiton/index
+   qualcomm/index
    rockchip/index
    sifive/index
    sipeed/index
diff --git a/doc/board/qualcomm/dragonboard410c.rst b/doc/board/qualcomm/dragonboard410c.rst
new file mode 100644
index 0000000000..d0de9dbcbc
--- /dev/null
+++ b/doc/board/qualcomm/dragonboard410c.rst
@@ -0,0 +1,45 @@
+.. SPDX-License-Identifier: GPL-2.0+
+.. sectionauthor:: Stephan Gerhold <stephan@gerhold.net>
+
+DragonBoard 410c
+================
+
+The DragonBoard 410c is a development board based on the Qualcomm APQ8016E SoC.
+More information can be found on the `96Boards product page`_.
+
+U-Boot can be used as a replacement for Qualcomm's original Android bootloader
+(a fork of Little Kernel/LK). Like LK, it is installed directly into the ``aboot``
+partition. Note that the U-Boot port used to be loaded as an Android boot image
+through LK. This is no longer the case, now U-Boot can replace LK entirely.
+
+.. _96Boards product page: https://www.96boards.org/product/dragonboard410c/
+
+Installation
+------------
+First, setup ``CROSS_COMPILE`` for aarch64. Then, build U-Boot for ``dragonboard410c``::
+
+  $ export CROSS_COMPILE=<aarch64 toolchain prefix>
+  $ make dragonboard410c_defconfig
+  $ make
+
+This will build ``u-boot.elf`` in the configured output directory.
+
+Although the DragonBoard 410c does not have secure boot set up by default,
+the firmware still expects firmware ELF images to be "signed". The signature
+does not provide any security in this case, but it provides the firmware with
+some required metadata.
+
+To "sign" ``u-boot.elf`` you can use e.g. `qtestsign`_::
+
+  $ ./qtestsign.py aboot u-boot.elf
+
+Then install the resulting ``u-boot-test-signed.mbn`` to the ``aboot`` partition
+on your device, e.g. with ``fastboot flash aboot u-boot-test-signed.mbn``.
+
+U-Boot should be running after a reboot (``fastboot reboot``).
+
+.. _qtestsign: https://github.com/msm8916-mainline/qtestsign
+
+Usage
+-----
+Press Volume Down during boot to enter Fastboot mode.
diff --git a/doc/board/qualcomm/index.rst b/doc/board/qualcomm/index.rst
new file mode 100644
index 0000000000..f7e0aa9298
--- /dev/null
+++ b/doc/board/qualcomm/index.rst
@@ -0,0 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Qualcomm
+========
+
+.. toctree::
+   :maxdepth: 2
+
+   dragonboard410c