[PATCH v5 0/8] clk: sunxi-ng: Add support for Allwinner A733 CCU and PRCM

[Norman Herms]

Hi Junhui,

> Thanks for pointing this out. This is indeed something I had not
> considered before. I will try enabling secure boot on my spare Cubie A7Z
> board and test it on actual hardware.

First a correction to my earlier reply in this thread. I answered
ChenYu's "presumably ... the secure/non-secure access bit isn't in
effect" with "yes" and quoted the UM note that these registers are
always non-secure in non-security mode. On our non-fused A7S boards
(SID + 0xA0 = 0, also when read from EL3) that does not hold for every
register. Read from EL3 with a test BL31, CCMU_SEC_SWITCH_REG is 0x7
after BL31's security setup, S_TWD_BGR_REG is 0x1 and the SPC status
registers are 0xffffffff, but from non-secure all of them read 0, and
non-secure writes to the first two do not stick. So secure filtering
is active on these parts even without the fuse.

> For the PLL and AHB/APB bus clock controls, the user manual indicates
> that the relevant security bits can be set by TF-A to allow the kernel
> to access these registers. This also appears to be how this has
> traditionally been handled on sunxi platforms. So I think we can do the
> same in TF-A for the A733.

That is what the BL31 on our boards does: PRCM_SEC_SWITCH_REG |= 0x7
and 0x7 to CCMU_SEC_SWITCH_REG at CCU + 0x1F00, as the vendor BL31
does. From EL3 both read 0 before and 0x7 after. The 0x1F00 versus
0x0F00 trap in the A733 TF-A port is in my first reply.

> For bus_r_twd_clk, I will test whether the register is indeed always
> secure. If it is, I think the clock can be removed from the kernel and
> left enabled by its hardware reset value or by the boot firmware.

On our non-fused boards it is already inaccessible from the non-secure
side (reads 0, writes ignored). From EL3 it reads 0x1, the UM default,
so the gate is open. Details are in my reply to ChenYu on patch 3/8,
which crossed with yours.

Same boards and caveats as before: AI agents (Claude) on my boards, a
report, not a Tested-by.

Thanks,
Norman