When source data width/burst and destination data width/burst are
different, data are packed or unpacked in DMA3 channel FIFO, using
CxTR1.PAM.
Data are pushed out from DMA3 channel FIFO when the destination burst
length (= data width * burst) is reached.
If the transfer is stopped before CxBR1.BNDT = 0, 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.

Prevent pack/unpack feature can be activated by setting bit 16 of DMA3
Transfer requirements bitfield (tr_conf) in device tree.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
---
 drivers/dma/stm32/stm32-dma3.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/dma/stm32/stm32-dma3.c b/drivers/dma/stm32/stm32-dma3.c
index b9470f783f98940a99addaeef6d0a8bc07b5c54b..f793eecd2c27ca17cedd5cabbaa1b1beca202039 100644
--- a/drivers/dma/stm32/stm32-dma3.c
+++ b/drivers/dma/stm32/stm32-dma3.c
@@ -221,6 +221,7 @@ enum stm32_dma3_port_data_width {
 #define STM32_DMA3_DT_BREQ		BIT(8) /* CTR2_BREQ */
 #define STM32_DMA3_DT_PFREQ		BIT(9) /* CTR2_PFREQ */
 #define STM32_DMA3_DT_TCEM		GENMASK(13, 12) /* CTR2_TCEM */
+#define STM32_DMA3_DT_NOPACK		BIT(16) /* CTR1_PAM */
 
 /* struct stm32_dma3_chan .config_set bitfield */
 #define STM32_DMA3_CFG_SET_DT		BIT(0)
@@ -622,6 +623,10 @@ static int stm32_dma3_chan_prep_hw(struct stm32_dma3_chan *chan, enum dma_transf
 		/* Set source (memory) data width and burst */
 		sdw = stm32_dma3_get_max_dw(chan->max_burst, sap_max_dw, len, src_addr);
 		sbl_max = stm32_dma3_get_max_burst(len, sdw, chan->max_burst);
+		if (!!FIELD_GET(STM32_DMA3_DT_NOPACK, tr_conf)) {
+			sdw = ddw;
+			sbl_max = dbl_max;
+		}
 
 		_ctr1 |= FIELD_PREP(CTR1_SDW_LOG2, ilog2(sdw));
 		_ctr1 |= FIELD_PREP(CTR1_SBL_1, sbl_max - 1);
@@ -652,6 +657,11 @@ static int stm32_dma3_chan_prep_hw(struct stm32_dma3_chan *chan, enum dma_transf
 		/* Set destination (memory) data width and burst */
 		ddw = stm32_dma3_get_max_dw(chan->max_burst, dap_max_dw, len, dst_addr);
 		dbl_max = stm32_dma3_get_max_burst(len, ddw, chan->max_burst);
+		if (!!FIELD_GET(STM32_DMA3_DT_NOPACK, tr_conf) ||
+		    ((_ctr2 & CTR2_PFREQ) && ddw > sdw)) { /* Packing to wider ddw not supported */
+			ddw = sdw;
+			dbl_max = sbl_max;
+		}
 
 		_ctr1 |= FIELD_PREP(CTR1_SDW_LOG2, ilog2(sdw));
 		_ctr1 |= FIELD_PREP(CTR1_SBL_1, sbl_max - 1);

-- 
2.25.1