4 files changed, 132 insertions, 2 deletions

diff --git a/docs/modules/ROOT/pages/faq.adoc b/docs/modules/ROOT/pages/faq.adoc
index 6cb7233c1..a074594cc 100644
--- a/docs/modules/ROOT/pages/faq.adoc
+++ b/docs/modules/ROOT/pages/faq.adoc
@@ -279,6 +279,14 @@ If you see errors that look like this:
 DMA: error on BDMA@1234ABCD channel 4
 ----
 
-You likely need to set up your linker script to define a special region for this area, and copy data to that region before using with BDMA.
+You need to set up your linker script to define a special region for this area, and copy data to that region before using with BDMA.
 
-TODO: show how to do that
+General steps:
+
+1. Find out which memory region BDMA has access to. You can get this information from the bus matrix and the memory mapping table in the STM32 datasheet.
+2. Add the memory region to `memory.x`, you can modify the generated one from https://github.com/embassy-rs/stm32-data-generated/tree/main/data/chips.
+3. You might need to modify `build.rs` to make cargo pick up the modified `memory.x`.
+4. In your code, access the defined memory region using `#[link_section = ".xxx"]`
+5. Copy data to that region before using BDMA.
+
+See link:/examples/stm32h7/src/bin/spi_bdma.rs[this example] for more details.
diff --git a/examples/stm32h7/build.rs b/examples/stm32h7/build.rs
index 8cd32d7ed..b9bb661f0 100644
--- a/examples/stm32h7/build.rs
+++ b/examples/stm32h7/build.rs
@@ -1,4 +1,34 @@
+//! This build script copies the `memory.x` file from the crate root into
+//! a directory where the linker can always find it at build time.
+//! For many projects this is optional, as the linker always searches the
+//! project root directory -- wherever `Cargo.toml` is. However, if you
+//! are using a workspace or have a more complicated build setup, this
+//! build script becomes required. Additionally, by requesting that
+//! Cargo re-run the build script whenever `memory.x` is changed,
+//! updating `memory.x` ensures a rebuild of the application with the
+//! new memory settings.
+
+use std::env;
+use std::fs::File;
+use std::io::Write;
+use std::path::PathBuf;
+
 fn main() {
+    // Put `memory.x` in our output directory and ensure it's
+    // on the linker search path.
+    let out = &PathBuf::from(env::var_os("OUT_DIR").unwrap());
+    File::create(out.join("memory.x"))
+        .unwrap()
+        .write_all(include_bytes!("memory.x"))
+        .unwrap();
+    println!("cargo:rustc-link-search={}", out.display());
+
+    // By default, Cargo will re-run a build script whenever
+    // any file in the project changes. By specifying `memory.x`
+    // here, we ensure the build script is only re-run when
+    // `memory.x` is changed.
+    println!("cargo:rerun-if-changed=memory.x");
+    
     println!("cargo:rustc-link-arg-bins=--nmagic");
     println!("cargo:rustc-link-arg-bins=-Tlink.x");
     println!("cargo:rustc-link-arg-bins=-Tdefmt.x");
diff --git a/examples/stm32h7/memory.x b/examples/stm32h7/memory.x
new file mode 100644
index 000000000..e5ab1f62c
--- /dev/null
+++ b/examples/stm32h7/memory.x
@@ -0,0 +1,14 @@
+MEMORY
+{
+    FLASH    : ORIGIN = 0x08000000, LENGTH = 2048K /* BANK_1 + BANK_2 */
+    RAM      : ORIGIN = 0x24000000, LENGTH = 512K  /* SRAM */
+    RAM_D3   : ORIGIN = 0x38000000, LENGTH = 64K   /* SRAM4 */
+}
+
+SECTIONS
+{
+    .ram_d3 :
+    {
+        *(.ram_d3)
+    } > RAM_D3
+}
\ No newline at end of file
diff --git a/examples/stm32h7/src/bin/spi_bdma.rs b/examples/stm32h7/src/bin/spi_bdma.rs
new file mode 100644
index 000000000..2fc37fc22
--- /dev/null
+++ b/examples/stm32h7/src/bin/spi_bdma.rs
@@ -0,0 +1,78 @@
+#![no_std]
+#![no_main]
+
+use core::fmt::Write;
+use core::str::from_utf8;
+
+use cortex_m_rt::entry;
+use defmt::*;
+use embassy_executor::Executor;
+use embassy_stm32::mode::Async;
+use embassy_stm32::time::mhz;
+use embassy_stm32::{peripherals, spi, Config};
+use heapless::String;
+use static_cell::StaticCell;
+use {defmt_rtt as _, panic_probe as _};
+
+// Defined in memory.x
+#[link_section = ".ram_d3"]
+static mut RAM_D3: [u8; 64 * 1024] = [0u8; 64 * 1024];
+
+#[embassy_executor::task]
+async fn main_task(mut spi: spi::Spi<'static, peripherals::SPI6, Async>) {
+    let read_buffer = unsafe { &mut RAM_D3[0..128] };
+    let write_buffer = unsafe { &mut RAM_D3[128..256] };
+    
+    for n in 0u32.. {
+        let mut write: String<128> = String::new();
+        core::write!(&mut write, "Hello DMA World {}!\r\n", n).unwrap();
+        let read_buffer = &mut read_buffer[..write.len()];
+        let write_buffer = &mut write_buffer[..write.len()];
+        // copy data to write_buffer which is located in D3 domain, accessable by BDMA
+        write_buffer.clone_from_slice(write.as_bytes());
+
+        spi.transfer(read_buffer, write_buffer).await.ok();
+        info!("read via spi+dma: {}", from_utf8(read_buffer).unwrap());
+    }
+}
+
+static EXECUTOR: StaticCell<Executor> = StaticCell::new();
+
+#[entry]
+fn main() -> ! {
+    info!("Hello World!");
+
+    let mut config = Config::default();
+    {
+        use embassy_stm32::rcc::*;
+        config.rcc.hsi = Some(HSIPrescaler::DIV1);
+        config.rcc.csi = true;
+        config.rcc.pll1 = Some(Pll {
+            source: PllSource::HSI,
+            prediv: PllPreDiv::DIV4,
+            mul: PllMul::MUL50,
+            divp: Some(PllDiv::DIV2),
+            divq: Some(PllDiv::DIV8), // used by SPI3. 100Mhz.
+            divr: None,
+        });
+        config.rcc.sys = Sysclk::PLL1_P; // 400 Mhz
+        config.rcc.ahb_pre = AHBPrescaler::DIV2; // 200 Mhz
+        config.rcc.apb1_pre = APBPrescaler::DIV2; // 100 Mhz
+        config.rcc.apb2_pre = APBPrescaler::DIV2; // 100 Mhz
+        config.rcc.apb3_pre = APBPrescaler::DIV2; // 100 Mhz
+        config.rcc.apb4_pre = APBPrescaler::DIV2; // 100 Mhz
+        config.rcc.voltage_scale = VoltageScale::Scale1;
+    }
+    let p = embassy_stm32::init(config);
+
+    let mut spi_config = spi::Config::default();
+    spi_config.frequency = mhz(1);
+
+    let spi = spi::Spi::new(p.SPI6, p.PA5, p.PA7, p.PA6, p.BDMA_CH1, p.BDMA_CH0, spi_config);
+
+    let executor = EXECUTOR.init(Executor::new());
+
+    executor.run(|spawner| {
+        unwrap!(spawner.spawn(main_task(spi)));
+    })
+}