How read AArch64 register in AArch32 mode - raspberry-pi

Question

i have a raspberry pi4 board that powered with Cortex-A72 processor. in this processor exist a special register that named "ID_AA64PFR0_EL1". document of Cortex-A72 explains:
To access the ID_AA64PFR0_EL1 in AArch64 state, read the register with:
MRS , ID_AA64PFR0_EL1; Read AArch64 Processor Feature Register 0
as document indicated with above command we could read ID_AA64PFR0_EL1 register in AArch64 mode. but official raspbian os only support AArch32 mode. so i want to know there is anyway that let me read ID_AA64PFR0_EL1 register in AArch32 mode?

Answer 1

Seems it's possible to get access to ID_AA64PFR0_EL1 in AArch32 mode.
According to "Architecture Reference Manual ARMv8"
ID_AA64PFR0_EL1, AArch64 Processor Feature Register 0 ...
Configurations
AArch64 System register ID_AA64PFR0_EL1 is architecturally mapped to
External register EDPFR
EDPFR, External Debug Processor Feature Register
EDPFR[31:0] can be accessed through the internal memory-mapped interface
According to same "Architecture Reference Manual ARMv8" and "ARM® Cortex®-A72 MPCore Processor, Technical Reference Manual"
To access the ID_AA64PFR0_EL1 in AArch64 state, read the register
with: MRS <Xt>, ID_AA64PFR0_EL1; Read AArch64 Processor Feature Register 0
The ID_AA64PFR0[31:0] can be accessed through the
memory-mapped interface and the external debug interface, offset
0xD20. The ID_AA64PFR0[63:32] can be accessed through the
memory-mapped interface and the external debug interface, offset
0xD24.
So you should check manufacturing documentation on your chip, and find out at what address "debug interface" is mapped.
After that read [addr + 0xd20] & [addr + 0xd24]
Enjoy.