On Sat, Jul 10, 2021 at 10:07:28PM +0300, Ramon Fried wrote:
On Wed, Jul 7, 2021 at 12:06 PM Stephan Gerhold stephan@gerhold.net wrote:
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:
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.
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:
qtestsign (https://github.com/msm8916-mainline/qtestsign) can be used as a "signing" tool for U-Boot and other firmware.
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
Related RFC with even more detailed explanations: https://lore.kernel.org/u-boot/YN2F1c926HFF+JA2@gerhold.net/
In my tests both U-Boot and Linux are fully functional with this patch. However, I would appreciate further testing, since my testing does likely not represent a typical usage scenario for the DragonBoard 410c.
When testing, please pick the following pending patch additionally: https://lore.kernel.org/u-boot/20210705121847.48432-1-stephan@gerhold.net/ on top of u-boot/master.
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Thanks Stephan, it looks very good. I started testing it, it builds correctly and I flashed and everything seems to work. I do have a problem with my TFTP setup, so I didn't boot Linux yet, but I will get to it, if it will boot successfully we can merge this one.
U-boot doesn't know how to boot qcom android kernel partitions, I'm not sure this is something I even want to start supporting in U-boot.
It's probably not too hard to support this (I actually boot the same Android boot images on both LK and U-Boot via "fastboot boot", because this is the workflow I'm used to). But I agree that anything else (i.e. reading the images from partitions) is not worth the effort.
Nico, do you think you can change the format of the BOOT partition to host a u-boot FIT image ?
Is a "boot" partition with a binary image even useful at all? I would expect that all typical Linux distributions except Android have a separate boot partition with a file system, so setting something up that works with the generic distro configuration (doc/README.distro) would be probably best. This would also avoid having to re-build the image just to change the cmdline for example.
Stephan