Led 和 Delay 抽象概念
现在，我将介绍两个高级抽象，我们将使用它们来实现LED轮盘应用程序。
auxiliary crate，aux5，公开了一个名为init的初始化函数。
调用此函数时，返回两个打包在元组中的值：Delay值和LedArray值。
Delay可以用于在指定的毫秒数内阻止程序。
LedArray是由八个Led组成的数组。每个Led代表F3板上的一个Led，并显示两种方法：
on和off分别用于打开或关闭Led。
让我们通过修改起始代码来尝试这两种抽象，如下所示：
#![deny(unsafe_code)] #![no_main] #![no_std] use aux5::{entry, Delay, DelayMs, LedArray, OutputSwitch}; #[entry] fn main() -> ! { let (mut delay, mut leds): (Delay, LedArray) = aux5::init(); let half_period = 500_u16; loop { leds[0].on().ok(); delay.delay_ms(half_period); leds[0].off().ok(); delay.delay_ms(half_period); } }
现在构建它：
cargo build
注意：在开始GDB会话之前，可能忘记重建程序；这种遗漏可能会导致非常混乱的调试会话。 为了避免这个问题，你可以只调用
cargo run而不是cargo build。cargo run命令将构建并 启动调试会话，确保您永远不会忘记重新编译程序。
现在，我们将运行并重复上一节中的闪烁过程，但使用新程序。我会让你输入cargo run，这将很快变得容易。 :)
注意：不要忘记在单独的终端上启动
openocd(调试器)。否则，target remote :3333将无法工作！
$ cargo run
Finished dev [unoptimized + debuginfo] target(s) in 0.01s
Running `arm-none-eabi-gdb -q ~/embedded-discovery/target/thumbv7em-none-eabihf/debug/led-roulette`
Reading symbols from ~/embedded-discovery/target/thumbv7em-none-eabihf/debug/led-roulette...
(gdb) target remote :3333
Remote debugging using :3333
led_roulette::__cortex_m_rt_main_trampoline () at ~/embedded-discovery/src/05-led-roulette/src/main.rs:7
7 #[entry]
(gdb) load
Loading section .vector_table, size 0x194 lma 0x8000000
Loading section .text, size 0x52c0 lma 0x8000194
Loading section .rodata, size 0xb50 lma 0x8005454
Start address 0x08000194, load size 24484
Transfer rate: 21 KB/sec, 6121 bytes/write.
(gdb) break main
Breakpoint 1 at 0x8000202: file ~/embedded-discovery/src/05-led-roulette/src/main.rs, line 7.
Note: automatically using hardware breakpoints for read-only addresses.
(gdb) continue
Continuing.
Breakpoint 1, led_roulette::__cortex_m_rt_main_trampoline ()
at ~/embedded-discovery/src/05-led-roulette/src/main.rs:7
7 #[entry]
(gdb) step
led_roulette::__cortex_m_rt_main () at ~/embedded-discovery/src/05-led-roulette/src/main.rs:9
9 let (mut delay, mut leds): (Delay, LedArray) = aux5::init();
(gdb)
好的，让我们一步一步看代码。这次，我们将使用next命令而不是step。不同的是，next命令将跳过函数调用，而不是进入它们内部。
(gdb) next
11 let half_period = 500_u16;
(gdb) next
13 loop {
(gdb) next
14 leds[0].on().ok();
(gdb) next
15 delay.delay_ms(half_period);
执行leds[0].on().ok() 语句后，您应该看到一个红色LED，指向北方，打开。
让我们继续浏览该程序：
(gdb) next
17 leds[0].off().ok();
(gdb) next
18 delay.delay_ms(half_period);
delay_ms调用将阻止程序半秒，但您可能没有注意到，因为next时间才能执行。然而，在跨过
leds[0].off()语句，您应该看到红色LED熄灭。
你已经可以猜到这个程序的作用了。使用continue命令让它不间断地运行。
(gdb) continue
Continuing.
现在，让我们做一些更有趣的事情。我们将使用GDB修改程序的行为。
首先，让我们按Ctrl+C停止无限循环。你很可能会在Led::on，Led::off 或delay_ms中的某个位置结束：
^C
Program received signal SIGINT, Interrupt.
0x08003434 in core::ptr::read_volatile<u32> (src=0xe000e010)
at ~/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:1053
在我的例子中，程序在read_volatile函数中停止了执行。GDB输出显示了一些有趣的信息：
core::ptr::read_volatile (src=0xe000e010)。这意味着该函数来自core crate，并且是用参数src = 0xe000e010调用的。
正如您所知，显示函数参数的更明确的方法是使用info args命令：
(gdb) info args
src = 0xe000e010
无论程序在何处停止，您都可以查看backtrace命令(简称bt) 的输出，了解它是如何停止的：
(gdb) backtrace
#0 0x08003434 in core::ptr::read_volatile<u32> (src=0xe000e010)
at ~/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:1053
#1 0x08002d66 in vcell::VolatileCell<u32>::get<u32> (self=0xe000e010) at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/vcell-0.1.3/src/lib.rs:33
#2 volatile_register::RW<u32>::read<u32> (self=0xe000e010) at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/volatile-register-0.2.0/src/lib.rs:75
#3 cortex_m::peripheral::SYST::has_wrapped (self=0x20009fa4)
at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/cortex-m-0.6.4/src/peripheral/syst.rs:136
#4 0x08003004 in stm32f3xx_hal::delay::{{impl}}::delay_us (self=0x20009fa4, us=500000)
at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/stm32f3xx-hal-0.5.0/src/delay.rs:58
#5 0x08002f3e in stm32f3xx_hal::delay::{{impl}}::delay_ms (self=0x20009fa4, ms=500)
at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/stm32f3xx-hal-0.5.0/src/delay.rs:32
#6 0x08002f80 in stm32f3xx_hal::delay::{{impl}}::delay_ms (self=0x20009fa4, ms=500)
at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/stm32f3xx-hal-0.5.0/src/delay.rs:38
#7 0x0800024c in led_roulette::__cortex_m_rt_main () at src/05-led-roulette/src/main.rs:15
#8 0x08000206 in led_roulette::__cortex_m_rt_main_trampoline () at src/05-led-roulette/src/main.rs:7
backtrace将打印从当前函数到main的函数调用跟踪。
回到我们的话题。要做我们要做的事情，首先，我们必须返回main函数。我们可以使用finish命令来完成。
该命令恢复程序执行，并在程序从当前函数返回后立即再次停止。我们得打调用几次。
(gdb) finish
Run till exit from #0 0x08003434 in core::ptr::read_volatile<u32> (src=0xe000e010)
at ~/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:1053
cortex_m::peripheral::SYST::has_wrapped (self=0x20009fa4)
at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/cortex-m-0.6.4/src/peripheral/syst.rs:136
136 self.csr.read() & SYST_CSR_COUNTFLAG != 0
Value returned is $1 = 5
(..)
(gdb) finish
Run till exit from #0 0x08002f3e in stm32f3xx_hal::delay::{{impl}}::delay_ms (self=0x20009fa4, ms=500)
at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/stm32f3xx-hal-0.5.0/src/delay.rs:32
0x08002f80 in stm32f3xx_hal::delay::{{impl}}::delay_ms (self=0x20009fa4, ms=500)
at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/stm32f3xx-hal-0.5.0/src/delay.rs:38
38 self.delay_ms(u32(ms));
(gdb) finish
Run till exit from #0 0x08002f80 in stm32f3xx_hal::delay::{{impl}}::delay_ms (self=0x20009fa4, ms=500)
at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/stm32f3xx-hal-0.5.0/src/delay.rs:38
0x0800024c in led_roulette::__cortex_m_rt_main () at src/05-led-roulette/src/main.rs:15
15 delay.delay_ms(half_period);
我们又回到了main。这里有一个局部变量：half_period
(gdb) print half_period
$3 = 500
现在，我们将使用set命令修改此变量：
(gdb) set half_period = 100
(gdb) print half_period
$5 = 100
如果您使用continue命令让程序再次自由运行，您可能会看到LED现在将以更快的速度闪烁，
但更可能的是闪烁速度没有改变。发生了什么？
让我们用Ctrl+C停止程序，然后在main:14处设置断点。
(gdb) continue
Continuing.
^C
Program received signal SIGINT, Interrupt.
core::cell::UnsafeCell<u32>::get<u32> (self=0x20009fa4)
at ~/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cell.rs:1711
1711 pub const fn get(&self) -> *mut T {
然后在main.rs:14上设置断点。并continue
(gdb) break main.rs:14
Breakpoint 2 at 0x8000236: file src/05-led-roulette/src/main.rs, line 14.
(gdb) continue
Continuing.
Breakpoint 2, led_roulette::__cortex_m_rt_main () at src/05-led-roulette/src/main.rs:14
14 leds[0].on().ok();
现在打开终端窗口，其长约80行，宽约170个字符（如果可能）。
注意：如果你不能打开那么大的终端，没问题，你会看到
--Type <RET> for more, q to quit, c to continue without paging--所以只需输入return，直到看到(gdb)提示符。然后滚动终端窗口以查看结果。
(gdb) disassemble /m
Dump of assembler code for function _ZN12led_roulette18__cortex_m_rt_main17h51e7c3daad2af251E:
8 fn main() -> ! {
0x08000208 <+0>: push {r7, lr}
0x0800020a <+2>: mov r7, sp
0x0800020c <+4>: sub sp, #64 ; 0x40
0x0800020e <+6>: add r0, sp, #32
9 let (mut delay, mut leds): (Delay, LedArray) = aux5::init();
0x08000210 <+8>: bl 0x8000302 <aux5::init>
0x08000214 <+12>: b.n 0x8000216 <led_roulette::__cortex_m_rt_main+14>
0x08000216 <+14>: add r0, sp, #32
0x08000218 <+16>: add r1, sp, #4
0x0800021a <+18>: ldmia.w r0, {r2, r3, r4, r12, lr}
0x0800021e <+22>: stmia.w r1, {r2, r3, r4, r12, lr}
0x08000222 <+26>: ldr r0, [sp, #52] ; 0x34
0x08000224 <+28>: ldr r1, [sp, #56] ; 0x38
0x08000226 <+30>: str r1, [sp, #28]
0x08000228 <+32>: str r0, [sp, #24]
0x0800022a <+34>: mov.w r0, #500 ; 0x1f4
10
11 let half_period = 500_u16;
0x0800022e <+38>: strh.w r0, [r7, #-2]
12
13 loop {
0x08000232 <+42>: b.n 0x8000234 <led_roulette::__cortex_m_rt_main+44>
0x08000234 <+44>: add r0, sp, #24
0x08000268 <+96>: b.n 0x8000234 <led_roulette::__cortex_m_rt_main+44>
14 leds[0].on().ok();
=> 0x08000236 <+46>: bl 0x80001ec <switch_hal::output::{{impl}}::on<stm32f3xx_hal::gpio::gpioe::PEx<stm32f3xx_hal::gpio::Output<stm32f3xx_hal::gpio::PushPull>>>>
0x0800023a <+50>: b.n 0x800023c <led_roulette::__cortex_m_rt_main+52>
0x0800023c <+52>: bl 0x8000594 <core::result::Result<(), core::convert::Infallible>::ok<(),core::convert::Infallible>>
0x08000240 <+56>: b.n 0x8000242 <led_roulette::__cortex_m_rt_main+58>
0x08000242 <+58>: add r0, sp, #4
0x08000244 <+60>: mov.w r1, #500 ; 0x1f4
15 delay.delay_ms(half_period);
0x08000248 <+64>: bl 0x8002f5c <stm32f3xx_hal::delay::{{impl}}::delay_ms>
0x0800024c <+68>: b.n 0x800024e <led_roulette::__cortex_m_rt_main+70>
0x0800024e <+70>: add r0, sp, #24
16
17 leds[0].off().ok();
0x08000250 <+72>: bl 0x800081a <switch_hal::output::{{impl}}::off<stm32f3xx_hal::gpio::gpioe::PEx<stm32f3xx_hal::gpio::Output<stm32f3xx_hal::gpio::PushPull>>>>
0x08000254 <+76>: b.n 0x8000256 <led_roulette::__cortex_m_rt_main+78>
0x08000256 <+78>: bl 0x8000594 <core::result::Result<(), core::convert::Infallible>::ok<(),core::convert::Infallible>>
0x0800025a <+82>: b.n 0x800025c <led_roulette::__cortex_m_rt_main+84>
0x0800025c <+84>: add r0, sp, #4
0x0800025e <+86>: mov.w r1, #500 ; 0x1f4
18 delay.delay_ms(half_period);
0x08000262 <+90>: bl 0x8002f5c <stm32f3xx_hal::delay::{{impl}}::delay_ms>
0x08000266 <+94>: b.n 0x8000268 <led_roulette::__cortex_m_rt_main+96>
End of assembler dump.
在上面的转储中，delay没有改变的原因是编译器认识到half_period没有改变，而是在delay的两个地方。
delay.delay_ms(half_period);被称为mov.w r1, #500。所以改变half_period的值没有任何作用！
0x08000244 <+60>: mov.w r1, #500 ; 0x1f4
15 delay.delay_ms(half_period);
0x08000248 <+64>: bl 0x8002f5c <stm32f3xx_hal::delay::{{impl}}::delay_ms>
(..)
0x0800025e <+86>: mov.w r1, #500 ; 0x1f4
18 delay.delay_ms(half_period);
0x08000262 <+90>: bl 0x8002f5c <stm32f3xx_hal::delay::{{impl}}::delay_ms>
该问题的一个解决方案是将half_period包装为Volatile如下所示。
#![deny(unsafe_code)]
#![no_main]
#![no_std]
use volatile::Volatile;
use aux5::{Delay, DelayMs, LedArray, OutputSwitch, entry};
#[entry]
fn main() -> ! {
let (mut delay, mut leds): (Delay, LedArray) = aux5::init();
let mut half_period = 500_u16;
let v_half_period = Volatile::new(&mut half_period);
loop {
leds[0].on().ok();
delay.delay_ms(v_half_period.read());
leds[0].off().ok();
delay.delay_ms(v_half_period.read());
}
}
编辑Cargo.toml文件，在[dependencies]地方添加volatile = "0.4.3"依赖。
[dependencies]
aux5 = { path = "auxiliary" }
volatile = "0.4.3"
通过使用Volatile的上述代码，您现在可以更改half_period，并且可以尝试不同的值。
以下是命令列表，后面是解释；# xxxx表示演示。
$ cargo run --target thumbv7em-none-eabihf # Compile and load the program into gdb
(gdb) target remote :3333 # Connect to STM32F3DISCOVERY board from PC
(gdb) load # Flash program
(gdb) break main.rs:16 # Set breakpoint 1 at top of loop
(gdb) continue # Continue, will stop at main.rs:16
(gdb) disable 1 # Disable breakpoint 1
(gdb) set print asm-demangle on # Enable asm-demangle
(gdb) disassemble /m # Disassemble main function
(gdb) continue # Led blinking on for 1/2 sec then off 1/2 sec
^C # Stop with Ctrl+C
(gdb) enable 1 # Enable breakpiont 1
(gdb) continue # Continue, will stop at main.rs:16
(gdb) print half_period # Print half_period result is 500
(gdb) set half_period = 2000 # Set half_period to 2000ms
(gdb) print half_period # Print half_period and result is 2000
(gdb) disable 1 # Disable breakpoint 1
(gdb) continue # Led blinking on for 2 secs then off 2 sec
^C # Stop with Ctrl+C
(gdb) quit # Quit gdb
关键的更改出现在源代码的第13、17和20行，您可以在反汇编中看到。在第13行，我们创建v_half_period，然后
read()第17行和第20行中的值。这意味着，当我们设置set half_period = 2000，led现在将打开2秒，然后关闭2秒。
$ cargo run --target thumbv7em-none-eabihf
Compiling led-roulette v0.2.0 (~/embedded-discovery/src/05-led-roulette)
Finished dev [unoptimized + debuginfo] target(s) in 0.18s
Running `arm-none-eabi-gdb -q ~/embedded-discovery/target/thumbv7em-none-eabihf/debug/led-roulette`
Reading symbols from ~/embedded-discovery/target/thumbv7em-none-eabihf/debug/led-roulette...
(gdb) target remote :3333
Remote debugging using :3333
led_roulette::__cortex_m_rt_main () at src/05-led-roulette/src/main.rs:16
16 leds[0].on().ok();
(gdb) load
Loading section .vector_table, size 0x194 lma 0x8000000
Loading section .text, size 0x5258 lma 0x8000194
Loading section .rodata, size 0xbd8 lma 0x80053ec
Start address 0x08000194, load size 24516
Transfer rate: 21 KB/sec, 6129 bytes/write.
(gdb) break main.rs:16
Breakpoint 1 at 0x8000246: file src/05-led-roulette/src/main.rs, line 16.
Note: automatically using hardware breakpoints for read-only addresses.
(gdb) continue
Continuing.
Breakpoint 1, led_roulette::__cortex_m_rt_main () at src/05-led-roulette/src/main.rs:16
16 leds[0].on().ok();
(gdb) disable 1
(gdb) set print asm-demangle on
(gdb) disassemble /m
Dump of assembler code for function _ZN12led_roulette18__cortex_m_rt_main17he1f2bc7990b13731E:
9 fn main() -> ! {
0x0800020e <+0>: push {r7, lr}
0x08000210 <+2>: mov r7, sp
0x08000212 <+4>: sub sp, #72 ; 0x48
0x08000214 <+6>: add r0, sp, #36 ; 0x24
10 let (mut delay, mut leds): (Delay, LedArray) = aux5::init();
0x08000216 <+8>: bl 0x800036a <aux5::init>
0x0800021a <+12>: b.n 0x800021c <led_roulette::__cortex_m_rt_main+14>
0x0800021c <+14>: add r0, sp, #36 ; 0x24
0x0800021e <+16>: add r1, sp, #8
0x08000220 <+18>: ldmia.w r0, {r2, r3, r4, r12, lr}
0x08000224 <+22>: stmia.w r1, {r2, r3, r4, r12, lr}
0x08000228 <+26>: ldr r0, [sp, #56] ; 0x38
0x0800022a <+28>: ldr r1, [sp, #60] ; 0x3c
0x0800022c <+30>: str r1, [sp, #32]
0x0800022e <+32>: str r0, [sp, #28]
0x08000230 <+34>: mov.w r0, #500 ; 0x1f4
11
12 let mut half_period = 500_u16;
0x08000234 <+38>: strh.w r0, [r7, #-6]
0x08000238 <+42>: subs r0, r7, #6
13 let v_half_period = Volatile::new(&mut half_period);
0x0800023a <+44>: bl 0x800033e <volatile::Volatile<&mut u16, volatile::access::ReadWrite>::new<&mut u16>>
0x0800023e <+48>: str r0, [sp, #68] ; 0x44
0x08000240 <+50>: b.n 0x8000242 <led_roulette::__cortex_m_rt_main+52>
14
15 loop {
0x08000242 <+52>: b.n 0x8000244 <led_roulette::__cortex_m_rt_main+54>
0x08000244 <+54>: add r0, sp, #28
0x08000288 <+122>: b.n 0x8000244 <led_roulette::__cortex_m_rt_main+54>
16 leds[0].on().ok();
=> 0x08000246 <+56>: bl 0x800032c <switch_hal::output::{{impl}}::on<stm32f3xx_hal::gpio::gpioe::PEx<stm32f3xx_hal::gpio::Output<stm32f3xx_hal::gpio::PushPull>>>>
0x0800024a <+60>: b.n 0x800024c <led_roulette::__cortex_m_rt_main+62>
0x0800024c <+62>: bl 0x80005fc <core::result::Result<(), core::convert::Infallible>::ok<(),core::convert::Infallible>>
0x08000250 <+66>: b.n 0x8000252 <led_roulette::__cortex_m_rt_main+68>
0x08000252 <+68>: add r0, sp, #68 ; 0x44
17 delay.delay_ms(v_half_period.read());
0x08000254 <+70>: bl 0x800034a <volatile::Volatile<&mut u16, volatile::access::ReadWrite>::read<&mut u16,u16,volatile::access::ReadWrite>>
0x08000258 <+74>: str r0, [sp, #4]
0x0800025a <+76>: b.n 0x800025c <led_roulette::__cortex_m_rt_main+78>
0x0800025c <+78>: add r0, sp, #8
0x0800025e <+80>: ldr r1, [sp, #4]
0x08000260 <+82>: bl 0x8002fc4 <stm32f3xx_hal::delay::{{impl}}::delay_ms>
0x08000264 <+86>: b.n 0x8000266 <led_roulette::__cortex_m_rt_main+88>
0x08000266 <+88>: add r0, sp, #28
18
19 leds[0].off().ok();
0x08000268 <+90>: bl 0x8000882 <switch_hal::output::{{impl}}::off<stm32f3xx_hal::gpio::gpioe::PEx<stm32f3xx_hal::gpio::Output<stm32f3xx_hal::gpio::PushPull>>>>
0x0800026c <+94>: b.n 0x800026e <led_roulette::__cortex_m_rt_main+96>
0x0800026e <+96>: bl 0x80005fc <core::result::Result<(), core::convert::Infallible>::ok<(),core::convert::Infallible>>
0x08000272 <+100>: b.n 0x8000274 <led_roulette::__cortex_m_rt_main+102>
0x08000274 <+102>: add r0, sp, #68 ; 0x44
20 delay.delay_ms(v_half_period.read());
0x08000276 <+104>: bl 0x800034a <volatile::Volatile<&mut u16, volatile::access::ReadWrite>::read<&mut u16,u16,volatile::access::ReadWrite>>
0x0800027a <+108>: str r0, [sp, #0]
0x0800027c <+110>: b.n 0x800027e <led_roulette::__cortex_m_rt_main+112>
0x0800027e <+112>: add r0, sp, #8
0x08000280 <+114>: ldr r1, [sp, #0]
0x08000282 <+116>: bl 0x8002fc4 <stm32f3xx_hal::delay::{{impl}}::delay_ms>
0x08000286 <+120>: b.n 0x8000288 <led_roulette::__cortex_m_rt_main+122>
End of assembler dump.
(gdb) continue
Continuing.
^C
Program received signal SIGINT, Interrupt.
0x080037b2 in core::cell::UnsafeCell<u32>::get<u32> (self=0x20009fa0) at ~/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cell.rs:1716
1716 }
(gdb) enable 1
(gdb) continue
Continuing.
Breakpoint 1, led_roulette::__cortex_m_rt_main () at src/05-led-roulette/src/main.rs:16
16 leds[0].on().ok();
(gdb) print half_period
$2 = 500
(gdb) disable 1
(gdb) continue
Continuing.
^C
Program received signal SIGINT, Interrupt.
0x08003498 in core::ptr::read_volatile<u32> (src=0xe000e010) at ~/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:1052
1052 unsafe { intrinsics::volatile_load(src) }
(gdb) enable 1
(gdb) continue
Continuing.
Breakpoint 1, led_roulette::__cortex_m_rt_main () at src/05-led-roulette/src/main.rs:16
16 leds[0].on().ok();
(gdb) print half_period
$3 = 500
(gdb) set half_period = 2000
(gdb) print half_period
$4 = 2000
(gdb) disable 1
(gdb) continue
Continuing.
^C
Program received signal SIGINT, Interrupt.
0x0800348e in core::ptr::read_volatile<u32> (src=0xe000e010) at ~/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:1046
1046 pub unsafe fn read_volatile<T>(src: *const T) -> T {
(gdb) q
Detaching from program: ~/embedded-discovery/target/thumbv7em-none-eabihf/debug/led-roulette, Remote target
Ending remote debugging.
[Inferior 1 (Remote target) detached]
问题! 如果你开始降低half_period的值会发生什么？在half_period的值是多少时，你再也看不到LED闪烁了吗？
现在，轮到你编写程序了。