Re: [PATCH v3 2/3] dmaengine: xilinx_dma: Enable transfer chaining for AXIDMA and MCDMA by removing idle restriction

["Pandey, Radhey Shyam" <radheys@amd.com>]

> Relax the idle check in xilinx_dma_start_transfer() and
> xilinx_mcdma_start_transfer() that prevented new transfers from being
> queued when the channel was busy, so scatter-gather transfers can be
> chained onto an in-flight transfer.
> 
> In scatter-gather mode, only update the CURDESC register when the active
> list is empty to avoid interfering with transfers already in progress.
> When the active list contains transfers, the hardware tail pointer
> extension mechanism handles chaining automatically via the descriptor
> next pointer chain, which is set up at channel allocation and preserved
> across descriptor recycling.
> 
> Direct (non-SG) mode has no descriptor queue: writing the BTT register
> launches a transfer immediately, so a new transfer must not be programmed
> while one is in flight. Keep those transfers serialized by retaining the
> idle check on the non-SG path. MCDMA always operates in scatter-gather
> mode, so it is unaffected.
> 
> Signed-off-by: Suraj Gupta <suraj.gupta2@amd.com>
> Co-developed-by: Srinivas Neeli <srinivas.neeli@amd.com>
> Signed-off-by: Srinivas Neeli <srinivas.neeli@amd.com>

Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Thanks!

> ---
>   drivers/dma/xilinx/xilinx_dma.c | 19 ++++++++++++-------
>   1 file changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/dma/xilinx/xilinx_dma.c b/drivers/dma/xilinx/xilinx_dma.c
> index ca396b709742..6e7b183cb499 100644
> --- a/drivers/dma/xilinx/xilinx_dma.c
> +++ b/drivers/dma/xilinx/xilinx_dma.c
> @@ -1580,7 +1580,14 @@ static void xilinx_dma_start_transfer(struct xilinx_dma_chan *chan)
>   		return;
>   	}
>   
> -	if (!chan->idle)
> +	/*
> +	 * Direct (non-SG) mode has no descriptor queue: writing the BTT
> +	 * register launches a transfer immediately, so a new transfer must
> +	 * not be programmed while one is in flight. Keep such transfers
> +	 * serialized. SG mode supports chaining onto a running transfer via
> +	 * tail-pointer extension, so it is allowed to proceed when busy.
> +	 */
> +	if (!chan->has_sg && !chan->idle)
>   		return;
>   
>   	head_desc = list_first_entry(&chan->pending_list,
> @@ -1599,7 +1606,7 @@ static void xilinx_dma_start_transfer(struct xilinx_dma_chan *chan)
>   		dma_ctrl_write(chan, XILINX_DMA_REG_DMACR, reg);
>   	}
>   
> -	if (chan->has_sg)
> +	if (chan->has_sg && list_empty(&chan->active_list))
>   		xilinx_write(chan, XILINX_DMA_REG_CURDESC,
>   			     head_desc->async_tx.phys);
>   	reg  &= ~XILINX_DMA_CR_DELAY_MAX;
> @@ -1660,9 +1667,6 @@ static void xilinx_mcdma_start_transfer(struct xilinx_dma_chan *chan)
>   	if (chan->err)
>   		return;
>   
> -	if (!chan->idle)
> -		return;
> -
>   	if (list_empty(&chan->pending_list))
>   		return;
>   
> @@ -1685,8 +1689,9 @@ static void xilinx_mcdma_start_transfer(struct xilinx_dma_chan *chan)
>   	dma_ctrl_write(chan, XILINX_MCDMA_CHAN_CR_OFFSET(chan->tdest), reg);
>   
>   	/* Program current descriptor */
> -	xilinx_write(chan, XILINX_MCDMA_CHAN_CDESC_OFFSET(chan->tdest),
> -		     head_desc->async_tx.phys);
> +	if (chan->has_sg && list_empty(&chan->active_list))
> +		xilinx_write(chan, XILINX_MCDMA_CHAN_CDESC_OFFSET(chan->tdest),
> +			     head_desc->async_tx.phys);
>   
>   	/* Program channel enable register */
>   	reg = dma_ctrl_read(chan, XILINX_MCDMA_CHEN_OFFSET);