[PATCH] dmaengine: cv1800b-dmamux: fix channel allocation order

[PATCH] dmaengine: cv1800b-dmamux: fix channel allocation order

[Anton D. Stavinskii]

The dmamux builds the free_maps list using llist_add(), which inserts
new nodes at the head. Using increasing channel indices causes the
first allocation to use DMA channel 7 while the DMA engine hands out
channel 0, leading to mismatched routing.

Reverse the channel index order so the first allocation gets channel 0.

Fixes: db7d07b5add4d ("dmaengine: add driver for Sophgo CV18XX/SG200X dmamux")
Signed-off-by: Anton D. Stavinskii <stavinsky@gmail.com>
---
 drivers/dma/cv1800b-dmamux.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/dma/cv1800b-dmamux.c b/drivers/dma/cv1800b-dmamux.c
index e900d6595617..825e1614051d 100644
--- a/drivers/dma/cv1800b-dmamux.c
+++ b/drivers/dma/cv1800b-dmamux.c
@@ -214,7 +214,7 @@ static int cv1800_dmamux_probe(struct platform_device *pdev)
 		}
 
 		init_llist_node(&tmp->node);
-		tmp->channel = i;
+		tmp->channel = MAX_DMA_CH_ID - i;
 		llist_add(&tmp->node, &data->free_maps);
 	}
 
-- 
2.43.0

Re: [PATCH] dmaengine: cv1800b-dmamux: fix channel allocation order

[Inochi Amaoto]

On Wed, Dec 10, 2025 at 11:30:12PM +0400, Anton D. Stavinskii wrote:
> The dmamux builds the free_maps list using llist_add(), which inserts
> new nodes at the head. Using increasing channel indices causes the
> first allocation to use DMA channel 7 while the DMA engine hands out
> channel 0, leading to mismatched routing.
> 
> Reverse the channel index order so the first allocation gets channel 0.
> 
> Fixes: db7d07b5add4d ("dmaengine: add driver for Sophgo CV18XX/SG200X dmamux")
> Signed-off-by: Anton D. Stavinskii <stavinsky@gmail.com>
> ---
>  drivers/dma/cv1800b-dmamux.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/dma/cv1800b-dmamux.c b/drivers/dma/cv1800b-dmamux.c
> index e900d6595617..825e1614051d 100644
> --- a/drivers/dma/cv1800b-dmamux.c
> +++ b/drivers/dma/cv1800b-dmamux.c
> @@ -214,7 +214,7 @@ static int cv1800_dmamux_probe(struct platform_device *pdev)
>  		}
>  
>  		init_llist_node(&tmp->node);
> -		tmp->channel = i;
> +		tmp->channel = MAX_DMA_CH_ID - i;
>  		llist_add(&tmp->node, &data->free_maps);
>  	}
>  
> -- 
> 2.43.0
> 

The problem is caused by the dw-axi-dmac driver, which allocate
channel dynamically. And the dma multiplexer can only routes
the interrupt statically, so it has the wrong information about
the channel enabled.

This patch does not fix the problem actually, I have sent a new
patch for it.

https://lore.kernel.org/all/20251212020504.915616-1-inochiama@gmail.com/

Regards,
Inochi