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TPIU: set_swo_baud_rate silently integer divisions, does not verify prescaler  #388

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TPIU: set_swo_baud_rate silently integer divisions, does not verify prescaler #388
#389
@tmplt

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@tmplt
tmplt
opened on Jan 10, 2022
Contributor

cortex-m/src/peripheral/tpiu.rs

Lines 105 to 113 in 92552c7

/// Sets the prescaler value for a wanted baud rate of the Serial
/// Wire Output (SWO) in relation to a given asynchronous refernce
/// clock rate.
#[inline]
pub fn set_swo_baud_rate(&mut self, ref_clk_rate: u32, baud_rate: u32) {
unsafe {
self.acpr.write((ref_clk_rate / baud_rate) - 1);
}
}

Experimentally, tpiu.set_swo_baud_rate(16_000_000, 115_200) works for me on an stm32, but instead of writing the expected (but invalid) 137.88, 137 is written. I theorize I just happen to be within a margin of error to kinda work, which would explain the invalid packets I get in rtic-scope/cargo-rtic-scope#18.

On another device, an atsame51n, tpiu.set_swo_baud_rate(120_000_000, 115_200) writes 1040 instead of 1040.66, and no trace output is seen on the SWO. Whether an invalid prescaler is the cause of this or just incorrect device setup remains to be seen.

In any case, the input arguments should yield an integer after division or otherwise fail. Preferably the function should also check what the largest implented prescaler value is.

Relevant section from ARMv7-M documentation attached below.

2022-01-10T18:10:04+01:00

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tmplt
added a commit that references this issue on Jan 10, 2022

tpiu: bitfield ACPR, verify set_swo_baud_rate args and ops

c2d9756
tmplt
linked a pull request that will close this issue on Jan 10, 2022
tmplt
added a commit that references this issue on Jan 10, 2022

tpiu: bitfield ACPR, verify set_swo_baud_rate args and ops

4d806cf
tmplt

tmplt commented on Jan 11, 2022

@tmplt
tmplt
on Jan 11, 2022
ContributorAuthor

openocd warns the user if a trace port frequency cannot be obtained from a clock input:

(gdb) monitor tpiu config external uart on 120000000 115200
Can not obtain 115200 trace port frequency from 120000000 TRACECLKIN frequency, using 115163 instead

and writes 1041 to SWOSCALER.

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        TPIU: `set_swo_baud_rate` silently integer divisions, does not verify prescaler · Issue #388 · rust-embedded/cortex-m