Issue with booting U-Boot from SD card, but using QSPI Flash for its Environment

[Michal Simek]

Hi,

On 6/27/22 05:01, David Antliff wrote:
> Hi,
> 
> I'm having some difficulty configuring U-Boot, in particular setting it up so that it can load from an SD card,
> but access its Environment from QSPI Flash.
> 
> First of all, is this even possible? Can the U-Boot environment be stored in a different location from where
> U-Boot was loaded?
> 
> Currently, I have a U-Boot image running from QSPI Flash on a Xilinx MPSoC ARM64 platform. I'm using Xilinx
> PetaLinux however I think I understand how this configures U-Boot "under the hood" so I'll try to refer to
> uboot.cfg​ as much as I can.
> 
> Along with all the defaults, I have set:
> 
> #define CONFIG_ENV_OFFSET 0x2E00000
> #define CONFIG_ENV_OVERWRITE 1
> #define CONFIG_CMD_ERASEENV 1
> 
> On this particular platform, there is a physical switch to switch the Xilinx first-stage bootloader (and ARM
> Trusted Firmware) between QSPI Flash to SD card. This platform also requires a so-called "BOOT.BIN", which
> contains the u-boot binary and other things (like the Xilinx FSBL, ARM Trusted Firmware, an FPGA image and
> other things), to be written to QSPI Flash, which I have done. This is all working correctly, and the serial console
> looks like this:
> 
> Xilinx Zynq MP First Stage Boot Loader
> Release 2021.2   Oct 13 2021  -  07:15:53
> NOTICE:  BL31: v2.4(release):xlnx_rebase_v2.4_2021.1_update1-23-g9188496b9
> NOTICE:  BL31: Built : 07:41:24, Oct 13 2021
> 
> 
> U-Boot 2021.01 (Oct 12 2021 - 09:28:42 +0000)
> 
> CPU:   ZynqMP
> Silicon: v3
> Model: ZynqMP ZCU208 RevA
> Board: Xilinx ZynqMP
> DRAM:  4 GiB
> PMUFW:  v1.1
> EL Level:       EL2
> Chip ID:        zu48dr
> NAND:  0 MiB
> MMC:   mmc at ff170000: 0
> Loading Environment from SPIFlash... SF: Detected mt25qu02g with page size 512 Bytes, erase size 128 KiB, total 512 MiB
> ...
> 
> As you can see, U-Boot loads from QSPI Flash and the Environment is also loaded from QSPI Flash. All good.
> 
> What I want to do is modify the system so that the earlier bootloader "BL31" loads U-Boot from the SD card. In my config I
> already have CONFIG_ENV_IS_IN_FAT set to 1. When I copy BOOT.BIN to the first, FAT partition on an SD card, and set my
> platform's boot mode to "SD Card" via the physical switch, the unit loads U-Boot from the SD card successfully, and the
> serial console shows the above but with:
> 
> Loading Environment from FAT... *** Warning - bad CRC, using default environment
> 
> It works, although this warning is expected because there is no Environment saved on the SD card yet.
> It does show that the SD card is being checked for the Environment though, so that's good.
> 
> Note that this is the same build that I used for the QSPI flash boot earlier, so I'm currently under the impression that
> U-Boot only looks for its Environment based on where it was loaded from, even if other locations are enabled.
> 
> The issue is that I would like to configure U-Boot to be loaded from the SD card, but to use QSPI for its Environment,
> so that we can store persistent U-Boot configuration on the board itself. Experimentally, it appears that U-Boot uses the
> medium it was loaded from as the place to look for its Environment, although I haven't been able to confirm this.
> 
> According to the U-Boot documentation I've read, disabling FAT as the default should be achievable by unsetting the
> following in u-boot.cfg:
> 
> CONFIG_ENV_IS_IN_FAT
> CONFIG_SYS_MMC_ENV_DEV
> CONFIG_ENV_FAT_DEVICE_AND_PART
> CONFIG_ENV_IS_IN_FAT
> CONFIG_ENV_FAT_FILE
> CONFIG_SYS_MMC_ENV_PART
> CONFIG_ENV_FAT_INTERFACE
> 
> Performing the same installation steps as above, to have this new U-Boot loaded from the SD card, results in the
> following output:
> 
> Xilinx Zynq MP First Stage Boot Loader
> Release 2021.2   Oct 13 2021  -  07:15:53
> NOTICE:  BL31: v2.4(release):xlnx_rebase_v2.4_2021.1_update1-23-g9188496b9
> NOTICE:  BL31: Built : 07:41:24, Oct 13 2021
> 
> And that's it! No more output. There is an LED on the board that goes from red to green, which I think indicates that
> the FPGA image within BOOT.BIN has been loaded into the gate array successfully, so I think the earlier boot loader
> has found BOOT.BIN. I have no reason therefore to think that U-Boot has not also been loaded, but if it has, it is
> generating no output. I tried increasing the debug level of U-Boot to 7 (DEBUG) in the hope of seeing something
> from U-Boot, but that makes no difference. It's also not booting Linux, which would normally happen next.
> 
> Can anyone shed any light on why making this change might result in this behaviour? Is there some other
> configuration option I need to set to ensure that U-Boot can run from the SD card when CONFIG_ENV_IS_IN_FAT
> and friends are not set? Or perhaps I can keep CONFIG_ENV_IS_IN_FAT etc. and there's some other way to instruct
> U-Boot to use the QSPI Flash for its environment, even though it was loaded from SD card?
> 

There are couple of things here. You are saying you want to U-Boot to be loaded 
from SD card. But FSBL you are using don't support this mode. It only supports 
that boot.bin has all binaries you want FSBL to load.

The second part where u-boot has it's own environment variables stores. By 
default as you properly described it is reading variables from location which 
corresponds with boot mode.
This is directed by
https://source.denx.de/u-boot/u-boot/-/blob/master/board/xilinx/zynqmp/zynqmp.c#L850

That's why if you want to change this setting you can simply return location you 
like. If it is FAT just simply change it to return ENVL_FAT.

We have been discussing this in past that would be the best to have better way 
how to configure it but none has found a time to implement it yet.

Thanks,
Michal

-- 
Michal Simek, Ing. (M.Eng), OpenPGP -> KeyID: FE3D1F91
w: www.monstr.eu p: +42-0-721842854
Maintainer of Linux kernel - Xilinx Microblaze
Maintainer of Linux kernel - Xilinx Zynq ARM and ZynqMP ARM64 SoCs
U-Boot custodian - Xilinx Microblaze/Zynq/ZynqMP/Versal SoCs