Allow DMA request zero for peripheral transfers

docs/api.md

@@ -90,6 +90,11 @@ request id on families that have one). The *value* is meaningless to
 supplied by whichever peripheral driver's own port knows which request line
 its hardware is wired to. Use `DMDMA_REQUEST_NONE` for
 `dmdma_direction_memory_to_memory` transfers, which need no request line.
+For a peripheral direction, the request is interpreted by the target port;
+zero is a valid selector on STM32F4/F7 and means hardware DMA channel 0.
+`DMDMA_REQUEST_NONE` is used only with memory-to-memory direction. The
+STM32 port continues to accept values 8 through 15 as aliases for channels
+0 through 7 so modules built against older releases keep working.
 
 ### `dmdma_transfer_config_t`
 

docs/dmdma.md

@@ -144,6 +144,11 @@ knows which controller/stream/request line its hardware is wired to opens
 that exact stream directly and drives it the same way, just with a real
 peripheral address and request line instead of `DMDMA_REQUEST_NONE`:
 
+For STM32F4/F7 peripheral transfers, pass request `0` for hardware channel 0.
+`DMDMA_REQUEST_NONE` (also zero) applies only to memory-to-memory transfers;
+the transfer direction makes the two uses unambiguous. The STM32 port still
+accepts legacy request `8` as a channel 0 alias.
+
 ```c
 dmdma_transfer_config_t rx = {
     .direction             = dmdma_direction_peripheral_to_memory,

include/dmdma_types.h

@@ -65,6 +65,8 @@ typedef enum
  * peripheral driver's own port (e.g. dmuart's stm32f7 port) knows which
  * request line its hardware is wired to. Use DMDMA_REQUEST_NONE for
  * dmdma_direction_memory_to_memory transfers, which need no request line.
+ * For a peripheral direction, selector zero is passed to the port: on
+ * STM32F4/F7 it is hardware channel zero. Direction determines the meaning.
  */
 typedef uint16_t dmdma_request_t;
 

src/dmdma.c

@@ -202,12 +202,8 @@ static int validate_transfer_config(dmdma_controller_t controller, const dmdma_t
         {
             return -EINVAL;
         }
-        if (config->request == DMDMA_REQUEST_NONE)
-        {
-            /* A peripheral direction with no request line can never fire -
-             * almost certainly a caller that forgot to set it. */
-            return -EINVAL;
-        }
+        /* A request selector belongs to the target port. Zero is a valid
+         * hardware channel on STM32F4/F7 and must reach that port. */
     }
 
     if (((uintptr_t)config->source_address % (uintptr_t)config->source_width) != 0)