fix fading issues

[TychoVrahe]

fixes #4392 fixes #4465

The slow fade in bootloader was due to function raising backlight was taking too long, waiting for display TE signal each time. Waiting for TE is removed as its no longer needed due to different mechanism using pending update - so now we will normally wait only once, after the display content is changed. In addition, the backlight fading function is rewritten to ensure constant fading time, at the possible expense of smoothness.

To mitigate the residual image issue, additional wait cycle is added when raising backlight.

As part of this fix i did i minor refactor, fading from python is now done by calling rust functions. The behavior of the fade is unified, so fading from python will behave a bit differently

Additional problem with fading is done before display refresh in progress layouts is fixed.

[github-actions]

core UI changes	device test	click test	persistence test
T2T1 Model T	test(screens) main(screens)	test(screens) main(screens)	test(screens) main(screens)
T3B1 Safe 3	test(screens) main(screens)	test(screens) main(screens)	test(screens) main(screens)
T3T1 Safe 5	test(screens) main(screens)	test(screens) main(screens)	test(screens) main(screens)
All	main(screens)

[matejcik]

trezor.ui imports backlight_fade symbol so changes to this file are not needed

(also we generally don't import trezorfoo modules directly in app code)

[TychoVrahe]

fixed 1c7bde1

[matejcik]

trezor.ui imports backlight_fade symbol so changes to this file are not needed

[TychoVrahe]

fixed 1c7bde1

[matejcik]

wondering if there's any point in keeping this testcase around? it's just a smoke test for existence of the function, we're not verifying any result here 🤷‍♀️

[TychoVrahe]

Probably not, we should either make something useful of it or remove it. I'd say its out of scope here though

[matejcik]

unwrap! please

here's a slightly tidier version of this function though:

#[cfg(feature = "backlight")]
pub fn fade_backlight_duration(target: u8, duration_ms: u32) {
    let target = target as i32;
    let current = backlight() as i32;
    let duration = Duration::from_millis(duration_ms);

    if animation_disabled() {
        set_backlight(target as u8);
        return;
    }

    let mut now = Instant::now();
    let start = now;
    let end = unwrap!(start.checked_add(duration));

    while now < end {
        let elapsed = unwrap!(now.checked_duration_since(start));
        let val = i32::lerp(current, target, elapsed / duration);
        set_backlight(val as u8);
        time::sleep(Duration::from_millis(1));
        now = Instant::now();
    }
    //account for imprecise rounding
    set_backlight(target as u8);
}

[TychoVrahe]

see below

[cepetr]

It looks like the problems have been solved. I just added a comment suggesting a minor improvement.

[cepetr]

There’s a small chance that the app might crash when the timer wraps from its maximum value to 0. How about using StopWatch instead of the timer here? It could slightly simplify the code and resolve this minor issue:

    let timer = Stopwatch::new_started();

    while timer.elapsed() < Duration::from_millis(duration_ms as _) {
        let elapsed = timer.elapsed().to_millis();
        let val = i32::lerp(current, target, elapsed as f32 / duration_ms as f32);
        set_backlight(val as u8);
        time::sleep(Duration::from_millis(1));
    }

[matejcik]

yeah, I suspected there's an existing code that could help here.

(also note that Durations support division and the result is a f32, see my suggested version of this function)

while you're touching this, please also convert this unwrap() to unwrap!

[TychoVrahe]

changed to the combo of your suggestions: 1c7bde1

i replaced that unwrap(), but note that there are unwrap()s in bunch of other places in our rust codebase, so probably some larger cleanup should take place. (would there be a way to automatically check for this?)