s-matyukevich/raspberry-pi-os

lesson2 can't see Exception level: 3 on the screen

garlic8668 opened this issue · 3 comments

garlic8668 commented

When I run the lesson01, I can see "Hello World" displayed on my screen normally, but when I use printf in the lesson02, my screen does not display "Exception level: 3" or anything.

subedisuman commented

Similar issue for me. I am using RPI4B and am able to run lesson01 with no problem after applying modifications suggested in #127. When I run lesson02 with lesson01's Boot.S, I can uart_send_string Hello World as in lesson01 plus I could also print current EL by uart_sending the int el as char but I was not able to see anything on screen with printf for the same. On the other hand, I was not even able to print anything on screen not even Hello World with Boot.S of lesson02.

ddnoodle commented

I met the same issue on RPi 4B module. Changing boot.S to the one in lesson 01 can see uart_send output, but no printf() output.
There must be something wrong in boot.S in lesson 02 on module RPi 4B. Who can help?

thanoskoutr commented

I am currently working on a bare metal OS for the Raspberry Pi 4 and I also had trouble, getting lesson 2 to work on the Pi 4. Firstly, I was able to get the execution level and print it to the serial console (without changing the boot.S), and it displayed EL2. I believe if you have kernel_old=1 set at the config.txt it might boots you at EL3, but I haven't checked it.
So I found that, with the new firmware, the kernel starts at EL2 and not EL3, as lesson 2 says. In order to change to EL1, I just removed the lines that change the EL3 registers and because the exception is taken at EL2, I save the SPSR_VALUE at the spsr_el2 register and save the return address at elr_el2.

Because I haven't followed the lessons from the beggining, I have a different linker script and approach at booting. I start my kernel at 0x80000 and not at 0x0 memory address.

So I believe, if you delete the kernel_old option from config.txt, that loads the kernel at memory address 0x0 and place the kernel at 0x80000 through the linker script. It should work.

Also, I setup the stack manually at 0x80000:

mov sp, #0x80000

Here is the changed master code, everything else in boot.S should be fine:

master:
	ldr x0, =SCTLR_VALUE_MMU_DISABLED
	msr sctlr_el1, x0

	ldr x0, =HCR_VALUE
	msr hcr_el2, x0

	ldr x0, =SPSR_VALUE
	msr spsr_el2, x0

	adr x0, el1_entry
	msr elr_el2, x0

	eret
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