AM62A7: Effects of Enabling Configuration of CPU Frequency in RT Linux

Kevin Peng

Expert 3752 points

Part Number: AM62A7

Tool/software:

Hi TI Experts,

Customer is working on AM62A RT-Linux SDK 9.2.

As you could see below, the default setting of CONFIG_CPU_FREQ is disabled.

Customer wants to enable it to improve their performance.

Customer knows that simply changing the CONFIG_CPU_FREQ = n to CONFIG_CPU_FREQ = y will work.

However, customer does not know why we make it as disable by default, they think maybe we know that there will be some effects of enabling it, so that's why we disable it as default.

Could you please let customer know the effects of of enabling CONFIG_CPU_FREQ = y ?

Thanks,

Kevin

over 1 year ago

0 Anshu Madwesh over 1 year ago

TI__Genius 17850 points

Hi Kevin,

To my understanding, RT Linux does not support CPUFreq drivers.

I've looped in the RT Linux expert to comment further, but they are currently out of office. Please ping this thread if you don't receive a response by next week.

Best Regards,

Anshu

0 arlen over 1 year ago

Prodigy 80 points

Additional content:

Add the following configuration, /sys/devices/system/cpu/cpu0/cpufreq is created successfully；

0 Kevin Peng over 1 year ago in reply to arlen

TI__Expert 3752 points

Hi TI Experts,

Please help check the setting provided by customer above.

They actually enabled it by doing so based on RT-Linux.

The only thing is that they are not sure about the effects of enabling it in RT-Linux.

Could you please provide some general suggestions for them?

Thanks,

Kevin

+1 Pekka Varis over 1 year ago in reply to Kevin Peng

TI__Mastermind 27050 points

Kevin Peng said:
The only thing is that they are not sure about the effects of enabling it in RT-Linux.

Could you please provide some general suggestions for them?

Generally frequency scaling should not be used with real-time. It will have severe impacts for worst case behaviour and possibly significantly add complexity to what is the worst case behaviour. See for example https://bootlin.com/doc/training/preempt-rt/preempt-rt-slides.pdf or https://people.mpi-sws.org/~bbb/papers/pdf/ospert13.pdf .

What is the goal the customer is trying to achieve with frequency scaling combined with real-time? In order to guarantee real-time performance you need to pick one operating point and optimize to meet the required deadlines.

Pekka

0 arlen over 1 year ago in reply to Pekka Varis

Prodigy 80 points

Hi Pekka

Thanks for your reply！

What is the goal the customer is trying to achieve with frequency scaling combined with real-time?

AI supplier want increase CPU freq to 1.4G ，this improves their overall performance;

1. Am62a7-linux-rt v:9.2 : the value of the default cpu frequency ? max cpu frequency?

2.Is it necessary to upgrade to 1.4G?

0 Pekka Varis over 1 year ago in reply to arlen

TI__Mastermind 27050 points

arlen said:
AI supplier want increase CPU freq to 1.4G ，this improves their overall performance;

As stated above with PREEMPT_RT you would not change the clock speed dynamically, this creates non-determinism. You design the system to meet the deadlines at a performance point and run at that performance point. In real-time, performance means meeting deadlines (either you are on time or not, being early is not typically higher performance, real-time means you are never late), there is no benefit in being faster.

arlen said:
2.Is it necessary to upgrade to 1.4G?

AM62A7 maximum clock speed depends on the exact part number (speed grade) and the core voltage. See https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1265906/am62a7-maximum-operating-frequency-of-am62a and datasheet https://www.ti.com/lit/gpn/am62a7 table 7-1 for more details. Basically with VDD_CORE at 0.85V you can run the A53 cores at 1.4GHz assuming you have the speed grade (letter S, T, U, V in the part number at position Z in red in AM62A74AZ).

arlen said:
1. Am62a7-linux-rt v:9.2 : the value of the default cpu frequency ? max cpu frequency?

The 9.2 SDK uses 1.2GHz A53 clock speed on the https://www.ti.com/tool/SK-AM62A-LP board. This is documented in for example https://software-dl.ti.com/processor-sdk-linux-rt/esd/AM62AX/latest/exports/docs/devices/AM62AX/linux/RT_Linux_Performance_Guide.html . You can change the clock speed, make sure you also adjust the VDD_CORE to 0.85V if you plan to run at 1.4GHz. It will probably work for short tests with typical silicon in a lab environment, but VDD_CORE as stated in the datasheet is required for product use.

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AM62A7: Effects of Enabling Configuration of CPU Frequency in RT Linux