When source data width/burst and destination data width/burst are
different, data are packed or unpacked in DMA3 channel FIFO.
Data are pushed out from DMA3 channel FIFO when the destination burst
length (= data width * burst) is reached.
If the channel is stopped before the transfer end, and if some bytes are
packed/unpacked in the DMA3 channel FIFO, these bytes are lost.
Indeed, DMA3 channel FIFO has no flush capability, only reset.
To avoid potential bytes lost, pack/unpack must be prevented by setting
memory data width/burst equal to peripheral data width/burst.
Memory accesses will be penalized. But it is the only way to avoid bytes
lost.

Some devices (e.g. cyclic RX like UART) need this, so add the possibility
to prevent pack/unpack feature, by setting bit 16 of the 'DMA transfer
requirements' bit mask.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
---
 Documentation/devicetree/bindings/dma/stm32/st,stm32-dma3.yaml | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Documentation/devicetree/bindings/dma/stm32/st,stm32-dma3.yaml b/Documentation/devicetree/bindings/dma/stm32/st,stm32-dma3.yaml
index 7fdc44b2e6467928622a5bb25d9e0c74bb1790ae..5484848735f8ac3d2050104bbab1d986e82ba6a7 100644
--- a/Documentation/devicetree/bindings/dma/stm32/st,stm32-dma3.yaml
+++ b/Documentation/devicetree/bindings/dma/stm32/st,stm32-dma3.yaml
@@ -96,6 +96,9 @@ properties:
                including the update of the LLI if any
           0x3: at channel level, the transfer complete event is generated at the
                end of the last LLI
+        -bit 16: Prevent packing/unpacking mode
+          0x0: pack/unpack enabled when source data width/burst != destination data width/burst
+          0x1: memory data width/burst forced to peripheral data width/burst to prevent pack/unpack
 
 required:
   - compatible

-- 
2.25.1

On Thu, 10 Oct 2024 16:27:51 +0200, Amelie Delaunay wrote:
> When source data width/burst and destination data width/burst are
> different, data are packed or unpacked in DMA3 channel FIFO.
> Data are pushed out from DMA3 channel FIFO when the destination burst
> length (= data width * burst) is reached.
> If the channel is stopped before the transfer end, and if some bytes are
> packed/unpacked in the DMA3 channel FIFO, these bytes are lost.
> Indeed, DMA3 channel FIFO has no flush capability, only reset.
> To avoid potential bytes lost, pack/unpack must be prevented by setting
> memory data width/burst equal to peripheral data width/burst.
> Memory accesses will be penalized. But it is the only way to avoid bytes
> lost.
> 
> Some devices (e.g. cyclic RX like UART) need this, so add the possibility
> to prevent pack/unpack feature, by setting bit 16 of the 'DMA transfer
> requirements' bit mask.
> 
> Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
> ---
>  Documentation/devicetree/bindings/dma/stm32/st,stm32-dma3.yaml | 3 +++
>  1 file changed, 3 insertions(+)
> 

Acked-by: Rob Herring (Arm) <robh@kernel.org>