JohnAmadis · Oct 11, 2026 · Oct 11, 2026
diff --git a/configs/board/nucleo-f767zi/usart3.ini b/configs/board/nucleo-f767zi/usart3.ini
@@ -28,5 +28,6 @@ parity=none
 stopbits=1
 flowcontrol=none
 instance=3
-rx_ring_size=256
+; ~44 ms at 921600 baud: what the DMA brings while the reader is not scheduled
+rx_ring_size=4096
 rx_ring_wait_mode=some_data
diff --git a/configs/board/nucleo-f767zi/usart6.ini b/configs/board/nucleo-f767zi/usart6.ini
@@ -28,5 +28,6 @@ parity=none
 stopbits=1
 flowcontrol=none
 instance=6
-rx_ring_size=256
+; ~44 ms at 921600 baud: what the DMA brings while the reader is not scheduled
+rx_ring_size=4096
 rx_ring_wait_mode=some_data
diff --git a/configs/board/stm32f746g-disco/usart1.ini b/configs/board/stm32f746g-disco/usart1.ini
@@ -28,5 +28,6 @@ parity=none
 stopbits=1
 flowcontrol=none
 instance=1
-rx_ring_size=256
+; ~44 ms at 921600 baud: what the DMA brings while the reader is not scheduled
+rx_ring_size=4096
 rx_ring_wait_mode=some_data
diff --git a/configs/board/stm32f746g-disco/usart6.ini b/configs/board/stm32f746g-disco/usart6.ini
@@ -28,5 +28,6 @@ parity=none
 stopbits=1
 flowcontrol=none
 instance=6
-rx_ring_size=256
+; ~44 ms at 921600 baud: what the DMA brings while the reader is not scheduled
+rx_ring_size=4096
 rx_ring_wait_mode=some_data
diff --git a/configs/board/stm32f769i-discovery/usart1.ini b/configs/board/stm32f769i-discovery/usart1.ini
@@ -28,5 +28,6 @@ parity=none
 stopbits=1
 flowcontrol=none
 instance=1
-rx_ring_size=256
+; ~44 ms at 921600 baud: what the DMA brings while the reader is not scheduled
+rx_ring_size=4096
 rx_ring_wait_mode=some_data
diff --git a/configs/board/stm32f769i-discovery/usart6.ini b/configs/board/stm32f769i-discovery/usart6.ini
@@ -28,5 +28,6 @@ parity=none
 stopbits=1
 flowcontrol=none
 instance=6
-rx_ring_size=256
+; ~44 ms at 921600 baud: what the DMA brings while the reader is not scheduled
+rx_ring_size=4096
 rx_ring_wait_mode=some_data
diff --git a/docs/configuration.md b/docs/configuration.md
@@ -19,6 +19,21 @@ All parameters are in the `[dmuart]` section:
 | `rx_ring_size` | integer | 256 | Capacity of the RX ring buffer, in bytes (0 disables it) |
 | `rx_ring_wait_mode` | string | "some_data" | RX ring read-wait behavior: "none" (return immediately), "some_data" (wait for at least one byte), "all_data" (wait for the full requested length) |
 
+### Receiving by DMA (STM32F7)
+
+On STM32F7 a UART with an RX ring receives by DMA: a circular 1 KiB buffer
+(from the `dma` heap when there is one - uncached DTCM -, else the D-cache
+is invalidated for it) that the USART's idle line and the DMA's half / end
+of the buffer drain into the ring. No byte is lost to an interrupt that
+runs late: the USART has no FIFO, at 921600 baud a byte comes every ~11 us.
+The stream is one of the two the UART can use (dmdma's leases); none free -
+the UART receives by interrupt, as without DMA.
+
+The ring is then what holds the bytes until the reader is scheduled: size
+it for that, not for a line. At 921600 baud 256 bytes are 2.8 ms; a reader
+reading byte by byte behind a busy thread (a display's rendering) misses
+them - the old ones are dropped. The F7 boards' consoles have 4096 (~44 ms).
+
 ## Examples
 
 ### Basic 115200 8N1

diff --git a/src/port/CMakeLists.txt b/src/port/CMakeLists.txt
@@ -41,11 +41,22 @@ dmod_add_library(${DMOD_MODULE_NAME} ${DMOD_MODULE_VERSION}
     ${COMMON_SOURCES}
 )
 
-dmod_link_modules(${DMOD_MODULE_NAME}
+# One dmod_link_modules() call: each call rewrites the target's version
+# requirements file.
+# stm32f7: RX by DMA - dmdma 0.4 (its lease API), dmheap (the buffer from
+# dmod-boot's "dma" heap, never cached).
+set(DMUART_PORT_MODULES
     dm_sw_ring
     dmclk_port
     dmosi
 )
+if(DMUART_MCU_SERIES STREQUAL "stm32f7")
+    list(APPEND DMUART_PORT_MODULES
+        dmdma@>=0.4
+        dmheap
+    )
+endif()
+dmod_link_modules(${DMOD_MODULE_NAME} ${DMUART_PORT_MODULES})
 
 target_include_directories(${DMOD_MODULE_NAME} BEFORE PRIVATE
     ${CMAKE_SOURCE_DIR}/include