While the Rockchip RK3588 processor is advertised as reaching 2.4 GHz, not all RK3588 chips may achieve this frequency. The keyword is PVTM (Process-Voltage-Temperature Monitor), and we’ll try to explain why it does, and why some of the RK3588 processors may only be clocked at about 2.3 GHz, while others will run fine at 2.4 GHz.
- Willy’s board: (Cortex-A76 cluster 1 @ 2,304 MHz, cluster 2 @ 2,352 MHz)
1234567# dmesg|grep cpu.*pvtm[ 2.606324] cpu cpu0: pvtm=1482[ 2.606542] cpu cpu0: pvtm-volt-sel=3[ 2.614206] cpu cpu4: pvtm=1722[ 2.618389] cpu cpu4: pvtm-volt-sel=5[ 2.626814] cpu cpu6: pvtm=1744[ 2.630998] cpu cpu6: pvtm-volt-sel=6
- CNXSoft board (Cortex-A76 cluster 1 @ 2,304 MHz, cluster 2 @ 2,304 MHz) :
123456[ 3.620281] cpu cpu0: pvtm=1490[ 3.620376] cpu cpu0: pvtm-volt-sel=4[ 3.627377] cpu cpu4: pvtm=1736[ 3.631320] cpu cpu4: pvtm-volt-sel=5[ 3.639103] cpu cpu6: pvtm=1732[ 3.643069] cpu cpu6: pvtm-volt-sel=5
- Thomas Kaiser (tkaiser) board: (Cortex-A76 cluster 1 @ 2,400 MHz, cluster 2 @ 2,400 MHz)
123456[ 3.117399] cpu cpu0: pvtm=1528[ 3.117491] cpu cpu0: pvtm-volt-sel=5[ 3.124529] cpu cpu4: pvtm=1785[ 3.128495] cpu cpu4: pvtm-volt-sel=7[ 3.136234] cpu cpu6: pvtm=1782[ 3.140173] cpu cpu6: pvtm-volt-sel=7
For reference, CPU 0 to 3 are Cortex-A55 cores, CPU 4-5 are two Cortex-A76 cores (cluster 1), and CPU 6-7 are two Cortex-A76 cores (cluster2). The frequencies reported above are Operating Performance Point (OPP) reported by the kernel, but the actual measured frequency in SBC-Bench.sh script may differ:
- CNXSoft board:
123456789Checking cpufreq OPP for cpu4-cpu5 (Cortex-A76):Cpufreq OPP: 2304 Measured: 2304 (2304.996/2304.969/2304.943)Cpufreq OPP: 2208 Measured: 2214 (2215.206/2215.036/2214.696)...Checking cpufreq OPP for cpu6-cpu7 (Cortex-A76):Cpufreq OPP: 2304 Measured: 2298 (2298.327/2298.300/2298.196)Cpufreq OPP: 2208 Measured: 2208 (2208.369/2208.369/2208.248)
- Thomas board
123456789Checking cpufreq OPP for cpu4-cpu5 (Cortex-A76):Cpufreq OPP: 2400 Measured: 2348 (2348.432/2348.405/2348.268) (-2.2%)Cpufreq OPP: 2208 Measured: 2185 (2185.642/2185.619/2185.571)Checking cpufreq OPP for cpu6-cpu7 (Cortex-A76):Cpufreq OPP: 2400 Measured: 2348 (2348.350/2348.268/2348.159) (-2.2%)Cpufreq OPP: 2208 Measured: 2185 (2185.548/2185.453/2185.311)
What’s really odd is that we have different OPP frequencies. Thomas’ Cortex-A76 cores are set up to 2,400 MHz, but are only measured to 2,348 MHz. It’s still better than the 2304 and 2,298 MHz measured on my system.
We’ll see if we can find more details about pvtm in the Rockchip RK3588 TRM (and SDK). It can indeed be found in Chapter 17 entitled “Process-Voltage-Temperature Monitor (PVTM)” of TRM Part 2.
As described in the technical reference manual:
The Process-Voltage-Temperature Monitor (PVTM) is used to monitor the chip performance variance caused by chip process, voltage, and temperature.
PVTM supports the following features:
- A clock oscillation ring is integrated and used to generate a clock like signal, the frequency of this clock is determined by the cell delay value of clock oscillation ring circuit.
- A calculation logic is used to measure the frequency of the clock oscillation ring.
- Follow PVTM blocks are supported:
- BIGCORE0_PVTM, used near A76_0/1
- BIGCORE1_PVTM, used near A76_2/3
- LITCORE_PVTM, used near DSU and A55_0/1/2/3
- NPU_PVTM, used near NPU
- GPU_PVTM, used near GPU
- PMU_PVTM, used near PMU
So that means not only the three CPU clusters (1x Cortex-A55 and 2x Cortex-A76) frequencies are impacted by the PVTM, but also the GPU and NPU frequencies, while for the PMU it appears to be used for low power modes as an alternative to a 32KHz clock source. The documentation further explains there are two methods of calculation (manual and auto):
A clock is generated by the oscillation ring and a frequency fixed clock clk_pvtm is used to calculate the cycles of the clock. Supposing the time period is 1s, then the clock period of oscillation ring clock is T= 1/clock_counter(s), the cell delay value is T/2.
For manual mode, user can only get one frequency result for a calculation.
For auto mode, user can set the calculation times, and get the maximum, minimum and average frequency during calculation. It also supports to generate an interrupt when the minimum or average frequency below a threshold. The threshold can be configured.
Every chip will be slightly different during manufacturing, and some may be able to reach 2.4 GHz while others not. The room temperature may also impact the performance. I’m based in the North of Thailand, and my room temperature is usually around 28°C, so it’s not impossible that my board can reach 2.4 GHz in winter (about 20°C in the morning), but is limited to about 2.3 GHz for the rest of the year…
The PVTM is probably used in conjunction with the PVTPLL (Chapter 18) which is “used to monitor the chip performance variance caused by chip process, voltage, and temperature, and generate a set of reference signals for adjusting the voltage of the chip.
I also wanted to check RK3588 Linux SDK on Gitlab as well, but I’ve just requested access from Rockchip, so we have to wait. Having said that the PVTM is not a new thing and I could find a patch submitted to mainline Linux in 2018 for the RK3288 processor, but I’ve just only noticed now. The Rockchip’s Linux CPUFreq driver documentation also mentions PVTM and how it assigns OPP values:
Rockchip’s CPUFreq driver attempts to read leakage value from eFuse and get frequency count from pvtm, then supplies the OPP framework with ‘prop’ information which is used to determine opp-microvolt-<name>property of OPPS when it is parsed by the OPP framework. This is based on operating-points-v2, but the driver can also create the “cpufreq-dt” platform_device to compatibility with operating-points.
Companies will sometimes create new part numbers for chips with the same functionality but different frequencies and Rockchip did that for RK3399K @ 2.0 GHz, RK3399 @ 1.8 GHz, and RK3399-T @ 1.5 GHz, but the small differences we’re seeing between Rockchip RK3588 processors probably did not warrant naming a new part, or it may come up later on.
Jean-Luc started CNX Software in 2010 as a part-time endeavor, before quitting his job as a software engineering manager, and starting to write daily news, and reviews full time later in 2011.