(low priority) demo-stm32h7-nucleo question - bsrr set_bit()

[SandyWalsh]

Looking at
https://github.com/oxidecomputer/hubris/blob/master/app/demo-stm32h7-nucleo/src/main.rs#L213-L216

I see the same set_bit() on both sides of the if statement.

Looking at the H7 reference manual (H755 but should be close enough)

12.3.5 I/O data bitwise handling

The bit set reset register (GPIOx_BSRR) is a 32-bit register which allows the application to
set and reset each individual bit in the output data register (GPIOx_ODR). The bit set reset
register has twice the size of GPIOx_ODR.

To each bit in GPIOx_ODR, correspond two control bits in GPIOx_BSRR: BS(i) and BR(i).
When written to 1, bit BS(i) sets the corresponding ODR(i) bit. When written to 1, bit BR(i)
resets the ODR(i) corresponding bit.

Writing any bit to 0 in GPIOx_BSRR does not have any effect on the corresponding bit in
GPIOx_ODR. If there is an attempt to both set and reset a bit in GPIOx_BSRR, the set
action takes priority.

I think this is the implementation of set_bit() being used
https://github.com/oxidecomputer/hubris/blob/master/drv/stm32xx-sys/src/main.rs#L368

If so, what's the objective of the context_switch() function here? Just to set the bit to 1 in all cases?

I'm just trying to understand what the "demo" is actually demoing :)

[labbott]

Look at commit

commit b09dfc01f132b78d3a1f02946b36e73ec4790fd6
Author: Cliff L. Biffle <[email protected]>
Date:   Thu Aug 11 11:15:55 2022 -0700

    kern: add context switch hook to profiling support.
    
    It's now possible, if somewhat involved, to keep track of which task is
    making syscalls / using the CPU.

This was added for profiling support

[SandyWalsh]

Thanks @labbott ... doh.

That explains the context_switch() function but I'm still not clear on what the value of the w closure is?

   gpioe.bsrr.write(|w| {
        if addr & 1 != 0 {
            w.bs2().set_bit();
        } else {
            w.br2().set_bit();
        }

the identical set_bit() calls (for each bit in the address)
I would have assumed one set_bit() and one clear_bit().
Perhaps I'm missing the purpose?

[dancrossnyc]

Those set two separate bits: "BS" vs "BR". w is the argument to the closure, and is a BSRR as in https://github.com/stm32-rs/stm32-rs-nightlies/blob/master/stm32f3/src/stm32f373/gpioa/bsrr.rs.

In the code snippet you highlighted above, if addr has the low bit set, set w's bs2, otherwise, set br2. Writing a 1 to bs2 (e.g., via bs2.set_bit()) sets pin 2, thus turning it on; similarly, writing a 1 to br2 resets the pin, thus turning it off.

[SandyWalsh]

Wow, I stared at that over and over and completely missed it. Under-caffeinated. Thanks for the 👀 @dancrossnyc

[dancrossnyc]

Happy to help!