Re: [PATCH 01/25] dmaengine: dw-edma: Drop dma_slave_config.direction field usage

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

On Thu, Mar 24, 2022 at 04:48:12AM +0300, Serge Semin wrote:
> The dma_slave_config.direction field usage in the DW eDMA driver has been
> introduced in the commit bd96f1b2f43a ("dmaengine: dw-edma: support local
> dma device transfer semantics"). Mainly the change introduced there was
> correct (indeed DEV_TO_MEM means using RD-channel and MEM_TO_DEV -
> WR-channel for the case of having eDMA accessed locally from
> CPU/Application side), but providing an additional
> MEM_TO_MEM/DEV_TO_DEV-based semantics was quite redundant if not to say
> potentially harmful (when it comes to removing the denoted field). First
> of all since the dma_slave_config.direction field has been marked as
> obsolete (see [1] and the structure dc [2]) and will be discarded in
> future, using it especially in a non-standard way is discouraged. Secondly
> in accordance with the commit denoted above the default
> dw_edma_device_transfer() semantics has been changed despite what it's
> message said. So claiming that the method was left backward compatible was
> wrong.
> 
> Anyway let's fix the problems denoted above and simplify the
> dw_edma_device_transfer() method by dropping the parsing of the
> DMA-channel direction field. Instead of having that implicit
> dma_slave_config.direction field semantic we can use the recently added
> DW_EDMA_CHIP_LOCAL flag to distinguish between the local and remote DW
> eDMA setups thus preserving both cases support. In addition to that an
> ASCII-figure has been added to clarify the complication out.
> 
> [1] Documentation/driver-api/dmaengine/provider.rst
> [2] include/linux/dmaengine.h: dma_slave_config.direction
> 
> Co-developed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
> 
> ---
> 
> In accordance with agreement with Frank and Manivannan this patch is
> supposed to be moved to the series:
> Link: https://lore.kernel.org/dmaengine/20220310192457.3090-1-Frank.Li@nxp.com/
> in place of the patch:
> [PATCH v5 6/9] dmaengine: dw-edma: Don't rely on the deprecated "direction" member
> Link: https://lore.kernel.org/dmaengine/20220310192457.3090-7-Frank.Li@nxp.com/
> ---
>  drivers/dma/dw-edma/dw-edma-core.c | 49 +++++++++++++++++++++---------
>  1 file changed, 34 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/dma/dw-edma/dw-edma-core.c b/drivers/dma/dw-edma/dw-edma-core.c
> index 5be8a5944714..e9e32ed74aa9 100644
> --- a/drivers/dma/dw-edma/dw-edma-core.c
> +++ b/drivers/dma/dw-edma/dw-edma-core.c
> @@ -339,21 +339,40 @@ dw_edma_device_transfer(struct dw_edma_transfer *xfer)
>  	if (!chan->configured)
>  		return NULL;
>  
> -	switch (chan->config.direction) {
> -	case DMA_DEV_TO_MEM: /* local DMA */
> -		if (dir == DMA_DEV_TO_MEM && chan->dir == EDMA_DIR_READ)
> -			break;
> -		return NULL;
> -	case DMA_MEM_TO_DEV: /* local DMA */
> -		if (dir == DMA_MEM_TO_DEV && chan->dir == EDMA_DIR_WRITE)
> -			break;
> -		return NULL;
> -	default: /* remote DMA */
> -		if (dir == DMA_MEM_TO_DEV && chan->dir == EDMA_DIR_READ)
> -			break;
> -		if (dir == DMA_DEV_TO_MEM && chan->dir == EDMA_DIR_WRITE)
> -			break;
> -		return NULL;
> +	/*
> +	 * Local Root Port/End-point              Remote End-point
> +	 * +-----------------------+ PCIe bus +----------------------+
> +	 * |                       |    +-+   |                      |
> +	 * |    DEV_TO_MEM   Rx Ch <----+ +---+ Tx Ch  DEV_TO_MEM    |
> +	 * |                       |    | |   |                      |
> +	 * |    MEM_TO_DEV   Tx Ch +----+ +---> Rx Ch  MEM_TO_DEV    |
> +	 * |                       |    +-+   |                      |
> +	 * +-----------------------+          +----------------------+
> +	 *
> +	 * 1. Normal logic:
> +	 * If eDMA is embedded into the DW PCIe RP/EP and controlled from the
> +	 * CPU/Application side, the Rx channel (EDMA_DIR_READ) will be used
> +	 * for the device read operations (DEV_TO_MEM) and the Tx channel
> +	 * (EDMA_DIR_WRITE) - for the write operations (MEM_TO_DEV).
> +	 *
> +	 * 2. Inverted logic:
> +	 * If eDMA is embedded into a Remote PCIe EP and is controlled by the
> +	 * MWr/MRd TLPs sent from the CPU's PCIe host controller, the Tx
> +	 * channel (EDMA_DIR_WRITE) will be used for the device read operations
> +	 * (DEV_TO_MEM) and the Rx channel (EDMA_DIR_READ) - for the write
> +	 * operations (MEM_TO_DEV).
> +	 *
> +	 * It is the client driver responsibility to choose a proper channel
> +	 * for the DMA transfers.
> +	 */

I think it'd be good to document this using some form in "enum dw_edma_dir"
declaration.

Thanks,
Mani

> +	if (chan->dw->chip->flags & DW_EDMA_CHIP_LOCAL) {
> +		if ((chan->dir == EDMA_DIR_READ && dir != DMA_DEV_TO_MEM) ||
> +		    (chan->dir == EDMA_DIR_WRITE && dir != DMA_MEM_TO_DEV))
> +			return NULL;
> +	} else {
> +		if ((chan->dir == EDMA_DIR_WRITE && dir != DMA_DEV_TO_MEM) ||
> +		    (chan->dir == EDMA_DIR_READ && dir != DMA_MEM_TO_DEV))
> +			return NULL;
>  	}
>  
>  	if (xfer->type == EDMA_XFER_CYCLIC) {
> -- 
> 2.35.1
>