[RFC] accel/rocket: DVFS on RK3588 - a hardware constraint, and some numbers

[Igor Paunovic]

Yesterday I wrote:

> My next step is to cherry-pick that commit into my BL31: if EL3
> also reads zero, the silicon answer wins.

Done this morning, and the answer is in: the firewall wins, and
Huseyin called it right.

Setup: d2d6928641ba cherry-picked onto the upstream TF-A v2.12 my
board runs (banner v2.12.0-10-g70d814213 verified over UART after
flashing). With that BL31, clk_get_rate() goes through EL3 reading
the PVTPLL status words directly.

NPU, from EL3: NPUGRF+0x24 reads a real value. clk_get_rate()
returns exactly 1000000000 at the 1 GHz OPP and exactly 700000000
at 700 MHz. At the same moment, the same register read from a
kernel module (EL1) still returns 0x00000000. Same register, same
instant: secure world sees the counter, non-secure gets zeros.
So the counter is alive in silicon, and yesterday's dead end was
the per-ip-core access restriction you suspected - not a missing
clock and not an unbonded counter.

GPU, from EL3, as cross-check under glmark2 load: five consecutive
reads gave 1048, 1053, 1047, 1054, 1049 MHz at the 1000 MHz OPP -
live jitter, matching the 1042 I measured from userspace mmap
yesterday. So EL3 reports raw measurements, which makes the NPU
result the more interesting one:

The NPU PVTPLL apparently LOCKS to its target (readback == nominal
to the MHz, at two operating points), while the GPU one free-runs
about 5% above nominal. Same register layout, same CON2 contents
as far as I can see - do you know what makes the two instances
behave differently?

Practical upshot for this series: with your TF-A commit in BL31,
plain clk_get_rate() gives the kernel measured rates with no GRF
access from the non-secure side at all. That is the clean
telemetry path for rocket DVFS, and one more reason your commit
earns its keep on RK3588.

One honest gap: I could not yet take the voltage-vs-frequency
curve (my devfreq module holds the NPU rail at 850 mV as a
regulator consumer, so undervolting from a second consumer is
refused). Future work.

Igor