st-trace: Synchronization issue

[simplerobot]

Discussed in #1248

st-trace's --clock parameter sets the system clock speed in a whole number of MHz. This can result in communication errors when using fractional MHz clock frequencies. We should allow setting the system clock frequency and trace frequency exactly using a common format.

I suggest we expect values provided with the "--clock" and "--trace" arguments to require a suffix of "Hz", "KHz", "MHz", or "GHz". If no suffix is provided we should continue to use a MHz default for "--clock" and a Hz default for "--trace" to maintain backward compatibility, but should display a warning message. We should also support fractional values, such as "7.384620MHz".

While digging into this issue, I also found the following related issues:

• There is a cosmetic bug that reads the TPI prescaler from the wrong register, causing us to print out the wrong clock speed configuration on startup.
• The STM32 chips appear to support a maximum trace speed of 1/5 of the system clock. There should be an error if the trace speed is set above this.

I'm happy to fix these. Does anyone have input on the format to specify clock frequencies?

Thanks,
John

[simplerobot]

Sorry, this is a dup of #1252 where I submitted a potential fix for these issues.