Correct DMA descriptor docs, math (#622)

CHANGELOG.md

@@ -39,6 +39,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Corrected the expected DMA descriptor counts (#622)
 - DMA is supported for SPI3 on ESP32-S3 (#507)
 - `change_bus_frequency` is now available on `SpiDma` (#529)
 - Fixed a bug where a GPIO interrupt could erroneously fire again causing the next `await` on that pin to instantly return `Poll::Ok` (#537)

esp-hal-common/src/dma/gdma.rs

@@ -390,7 +390,8 @@ macro_rules! impl_channel {
             impl [<ChannelCreator $num>] {
                 /// Configure the channel for use
                 ///
-                /// Descriptors should be sized as (BUFFERSIZE / 4092) * 3
+                /// Descriptors should be sized as `((BUFFERSIZE + 4091) / 4092) * 3`. I.e., to
+                /// transfer buffers of size `1..=4092`, you need 3 descriptors.
                 pub fn configure<'a>(
                     self,
                     burst_mode: bool,

esp-hal-common/src/dma/mod.rs

@@ -1,6 +1,7 @@
 //! Direct Memory Access Commons
 //!
-//! Descriptors should be sized as (BUFFERSIZE / 4092) * 3
+//! Descriptors should be sized as `((BUFFERSIZE + 4091) / 4092) * 3`. I.e., to
+//! transfer buffers of size `1..=4092`, you need 3 descriptors.
 
 use core::{marker::PhantomData, sync::atomic::compiler_fence};
 
@@ -347,7 +348,7 @@ where
             return Err(DmaError::InvalidDescriptorSize);
         }
 
-        if self.descriptors.len() / 3 < len / CHUNK_SIZE {
+        if self.descriptors.len() / 3 < (len + CHUNK_SIZE - 1) / CHUNK_SIZE {
             return Err(DmaError::OutOfDescriptors);
         }
 
@@ -647,7 +648,7 @@ where
             return Err(DmaError::InvalidDescriptorSize);
         }
 
-        if self.descriptors.len() / 3 < len / CHUNK_SIZE {
+        if self.descriptors.len() / 3 < (len + CHUNK_SIZE - 1) / CHUNK_SIZE {
             return Err(DmaError::OutOfDescriptors);
         }