CH32V307 : FPU + FreeRTOS

Toolchain/Clang

 Time to enable the FPU!

I've modified the  LLVM-embedded-toolchain-for-rv32 so that it builds both soft-ftp and hard-fp clang runtimes (it is a bit clunky but works).


FreeRTOS

WCH implementation

The FreeRTOS implementation saves this on the task stack : 

  • MEPC
  • [FPU]
  • Normal regs
  • MStatus

[FPU] depends on a compilation switch. 

It contains either all the FPU registers (the 32 of them) or nothing. Always.

This is problematic.

Saving all those registers is hitting the stack hard and it takes time.

Furthermore, that happens for all task switches  even if the FPU is not used.

Conversely, if you dont save the FPU registers and use the FPU in 2 different tasks, you'll get data corruption.

Naive improvement

I implemented a very naive change using the FS bits in mstatus.

These FS bits can have 2 interesting values : 

  • FPU never used (Off, original, clean)
  • FPU dirty (i.e. used at some point).

So, when the FS bits are "not used", you can skip saving/restoring the FPU registers for that task.

When they are "dirty", you need to save/restore the FPU registers

It is a bit naive, but is acceptable imho.

That involved changing the register layout a bit on the stack to this :

  • MEPC
  • MStatus
  • [FPU]
  • Normal regs
We have Mstatus available at a fixed position, so it can be checked to see the status of the FS bits.
The FPU registers are only saved to/out of the stack if the FS bits are dirty.

So we get the best of both worlds by paying a very small CPU overhead, mostly due to my ignorance of proper riscv assembly :
  • ~ 0 impact when you dont use the FPU
  • No corruption when you use them, but you pay the price
It can probably be better, as it is now it will get dirty if you use them only once in a task, but it is a good start.

Comments

Post a Comment

Popular posts from this blog

Component tester with STM32 : Part 1 ADC, Resistor

Image
I was always  fascinated by the so call "transistor testers" that identify and report pin out, features of pretty much whatever you throw at it. They are based on Atmel 328p chip, the same as arduino nano. So why not do the same thing with a STM32/Bluepill ? The STM32 is faster, has a better ADC accuracy, more memory, can do float etc... and is not more expensive. Warning : I'm doing this for fun & to learn, it might contain error & plain misunderstanding So, i watched a couple of video on the general principle on how they work. This is not a "port" but a rewrite from ~ scratch. When something is weird, i check what the original one does for comparison. Measuring resistance So first thing first :  Resistance & Capacitance. These 2 will unlock the other ones. So how do we measure resistance ? What we do is a resistor divider with a know resistance value: Measure = Vcc*(RtoTest/(RtoTest+Rknown)) As far as the ADC goes, it means ADC=
Read more

Fixing the INA3221

Image
It 's not too complicated to change the board we have seen  previously . You 'll need to cut some tracks, i did it with a dremel and i'm not very handy, so you'll do better. Red =Cut tracks Blue = Add wire 1- You'll need to separate the bottom connector from the ground, cut the 3 connectors BOTH SIDES. 2- Remove the small red area below the middle resistor to isolate each channels from each other Last, connect right side of each connector to the now isolated pad. Mine looks like this after the operation (ugly i know): Be wary of the small track on the left connector that leads to the left side of channel3 resistor. I cut it while isolating the right pad. Now it works, we can go back to the charger.
Read more

INA3221, weird wiring

Image
Continuing my "smart" battery charger, i've finally received the INA3221 module. It is the purple one you can find on ebay for ~ 5 bucks I soldered a couple of pins to be able to connect it so that i could test it and something immediately caught my attention. It is supposed to be a high side current/voltage sensor, typically in a setup like that : So you can measure the voltage supplied to the load, and the current going through it. Now, let's have a look at the module, look at the top of the following pic, where the sense connectors are. CH3/GND/CH2/GND/CH1/GND ? Wait ? What ? Yes, all the IN- pins are connected to ground, so it is a bottom current only sensor. Additionally, the - side of the shunts are ALSO connected to the ground, no only the connectors. The actual layout is like that : That raises two problems :  The voltage is meaningless as we are only measuring Current x ShuntValue, not the supply volt
Read more