[PATCH RESEND] dmaengine: stm32-dma: configure next sg only if there are more than 2 sgs

[PATCH RESEND] dmaengine: stm32-dma: configure next sg only if there are more than 2 sgs

[Amelie Delaunay]

DMA operates in Double Buffer Mode (DBM) when the transfer is cyclic and
there are at least two periods.
When DBM is enabled, the DMA toggles between two memory targets (SxM0AR and
SxM1AR), indicated by the SxSCR.CT bit (Current Target).
There is no need to update the next memory address if two periods are
configured, as SxM0AR and SxM1AR are already properly set up before the
transfer begins in the stm32_dma_start_transfer() function.
This avoids unnecessary updates to SxM0AR/SxM1AR, thereby preventing
potential Transfer Errors. Specifically, when the channel is enabled,
SxM0AR and SxM1AR can only be written if SxSCR.CT=1 and SxSCR.CT=0,
respectively. Otherwise, a Transfer Error interrupt is triggered, and the
stream is automatically disabled.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
---
 drivers/dma/stm32/stm32-dma.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/dma/stm32/stm32-dma.c b/drivers/dma/stm32/stm32-dma.c
index 917f8e9223739af853e492d97cecac0e95e0aea3..0e39f99bce8be8c38fe33dd0246012910243d831 100644
--- a/drivers/dma/stm32/stm32-dma.c
+++ b/drivers/dma/stm32/stm32-dma.c
@@ -744,7 +744,7 @@ static void stm32_dma_handle_chan_done(struct stm32_dma_chan *chan, u32 scr)
 		/* cyclic while CIRC/DBM disable => post resume reconfiguration needed */
 		if (!(scr & (STM32_DMA_SCR_CIRC | STM32_DMA_SCR_DBM)))
 			stm32_dma_post_resume_reconfigure(chan);
-		else if (scr & STM32_DMA_SCR_DBM)
+		else if (scr & STM32_DMA_SCR_DBM && chan->desc->num_sgs > 2)
 			stm32_dma_configure_next_sg(chan);
 	} else {
 		chan->busy = false;

---
base-commit: de266931dd996fc2cb8ee8b5d12e39ea463e3f36
change-id: 20250605-stm32_dma_dbm_fix-3b86271d54d4

Best regards,
-- 
Amelie Delaunay <amelie.delaunay@foss.st.com>

Re: [PATCH RESEND] dmaengine: stm32-dma: configure next sg only if there are more than 2 sgs

[Vinod Koul]

On Tue, 24 Jun 2025 09:31:37 +0200, Amelie Delaunay wrote:
> DMA operates in Double Buffer Mode (DBM) when the transfer is cyclic and
> there are at least two periods.
> When DBM is enabled, the DMA toggles between two memory targets (SxM0AR and
> SxM1AR), indicated by the SxSCR.CT bit (Current Target).
> There is no need to update the next memory address if two periods are
> configured, as SxM0AR and SxM1AR are already properly set up before the
> transfer begins in the stm32_dma_start_transfer() function.
> This avoids unnecessary updates to SxM0AR/SxM1AR, thereby preventing
> potential Transfer Errors. Specifically, when the channel is enabled,
> SxM0AR and SxM1AR can only be written if SxSCR.CT=1 and SxSCR.CT=0,
> respectively. Otherwise, a Transfer Error interrupt is triggered, and the
> stream is automatically disabled.
> 
> [...]

Applied, thanks!

[1/1] dmaengine: stm32-dma: configure next sg only if there are more than 2 sgs
      commit: e19bdbaa31082b43dab1d936e20efcebc30aa73d

Best regards,
-- 
~Vinod