Re: [PATCH v2 1/5] dmaengine: Add DMA_SG support for multi-buffer scatter-gather transfers

On 03-08-26, 16:01, Sumit Kumar wrote:
> A client that needs to copy several independent, non-contiguous memory
> regions in one operation currently has to submit one DMA_MEMCPY
> transaction per region, each with its own completion interrupt, even
> when every region is known up front and the copies could be described
> as a single hardware transaction.
> 
> Partially reintroduce the DMA_SG transaction type and device_prep_dma_sg()
> API that was removed in commit c678fa66341c ("dmaengine: remove DMA_SG as
> it is dead code in kernel"). Only the core API is restored here.
> 
> The API accepts separate source and destination scatter-gather lists,
> where entry i of the source list is transferred to entry i of the
> destination list. This allows multiple independent (src[i] -> dst[i])
> transfers to be batched into a single DMA transaction instead of N
> separate submissions, reducing submission and interrupt overhead.
> 
> DMA_SG is a memcpy-class operation: both endpoints are memory buffers,
> and neither DMA address is a FIFO-style peripheral register. The source
> and destination scatter-gather lists must contain the same number of
> entries; providers reject requests where the entry counts differ.
> 
> Restore the DMA_SG entry in
> Documentation/driver-api/dmaengine/provider.rst and add
> CHECK_CAP(dma_sg, DMA_SG) to dma_async_device_register() to validate
> that drivers setting the capability provide the corresponding function
> pointer.
> 
> Signed-off-by: Sumit Kumar <[email protected]>
> ---
>  Documentation/driver-api/dmaengine/provider.rst | 21 +++++++++++++++
>  drivers/dma/dmaengine.c                         |  1 +
>  include/linux/dmaengine.h                       | 35 +++++++++++++++++++++++++
>  3 files changed, 57 insertions(+)
> 
> diff --git a/Documentation/driver-api/dmaengine/provider.rst b/Documentation/driver-api/dmaengine/provider.rst
> index f4ed98f701c918ff81bc674845880f8d01efbf1d..638e4b83e9a2f90c056111dbdd7572a4ed0f536d 100644
> --- a/Documentation/driver-api/dmaengine/provider.rst
> +++ b/Documentation/driver-api/dmaengine/provider.rst
> @@ -210,6 +210,27 @@ Currently, the types available are:
>    - Used by the client drivers to register a callback that will be
>      called on a regular basis through the DMA controller interrupt
>  
> +- DMA_SG
> +
> +  - The device supports memory to memory scatter-gather transfers
> +    using paired source and destination scatter-gather lists, where
> +    entry ``i`` of the source list is transferred to entry ``i`` of
> +    the destination list in a single DMA transaction.
> +
> +  - The source and destination scatter-gather lists must contain the
> +    same number of entries; providers reject (return NULL for) requests
> +    where the entry counts differ. Providers that walk the two lists in
> +    lockstep pair them entry-by-entry as passed in, so clients that
> +    DMA-map the lists must ensure the mapped segmentation stays aligned
> +    between the two lists (for example by not relying on the DMA layer
> +    to merge entries of one list but not the other).
> +
> +  - Unlike DMA_MEMCPY, neither the source nor destination is a
> +    FIFO-style peripheral register; both are memory buffers. Multiple
> +    independent (src[i] -> dst[i]) copies are submitted as a single
> +    DMA transaction, reducing submission and interrupt overhead
> +    compared to N separate DMA_MEMCPY operations.
> +
>  - DMA_PRIVATE
>  
>    - The devices only supports slave transfers, and as such isn't
> diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
> index 6ffd8bd82154af2af2807d1c8b7ae7475eab56d3..9e790b9f165063d696438ace6370df98dffb8a32 100644
> --- a/drivers/dma/dmaengine.c
> +++ b/drivers/dma/dmaengine.c
> @@ -1211,6 +1211,7 @@ int dma_async_device_register(struct dma_device *device)
>  	CHECK_CAP(dma_pq_val,      DMA_PQ_VAL);
>  	CHECK_CAP(dma_memset,      DMA_MEMSET);
>  	CHECK_CAP(dma_interrupt,   DMA_INTERRUPT);
> +	CHECK_CAP(dma_sg,          DMA_SG);
>  	CHECK_CAP(dma_cyclic,      DMA_CYCLIC);
>  	CHECK_CAP(interleaved_dma, DMA_INTERLEAVE);
>  
> diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> index fe33a20abc6146d539670e0e6fe6c9d27d96aa2a..61aa72149f5d5cf1828b89d13d0b35f1a12bb000 100644
> --- a/include/linux/dmaengine.h
> +++ b/include/linux/dmaengine.h
> @@ -65,6 +65,7 @@ enum dma_transaction_type {
>  	DMA_COMPLETION_NO_ORDER,
>  	DMA_REPEAT,
>  	DMA_LOAD_EOT,
> +	DMA_SG,
>  /* last transaction type for creation of the capabilities mask */
>  	DMA_TX_TYPE_END,
>  };
> @@ -848,6 +849,7 @@ struct dma_filter {
>   *	The function takes a buffer of size buf_len. The callback function will
>   *	be called after period_len bytes have been transferred.
>   * @device_prep_interleaved_dma: Transfer expression in a generic way.
> + * @device_prep_dma_sg: prepares a memory to memory scatter-gather operation
>   * @device_caps: May be used to override the generic DMA slave capabilities
>   *	with per-channel specific ones
>   * @device_config: Pushes a new configuration to a channel, return 0 or an error
> @@ -954,6 +956,11 @@ struct dma_device {
>  	struct dma_async_tx_descriptor *(*device_prep_interleaved_dma)(
>  		struct dma_chan *chan, struct dma_interleaved_template *xt,
>  		unsigned long flags);
> +	struct dma_async_tx_descriptor *(*device_prep_dma_sg)
> +		(struct dma_chan *chan,
> +		 struct scatterlist *dst_sg, unsigned int dst_nents,
> +		 struct scatterlist *src_sg, unsigned int src_nents,
> +		 unsigned long flags);
>  
>  	void (*device_caps)(struct dma_chan *chan, struct dma_slave_caps *caps);
>  	int (*device_config)(struct dma_chan *chan, struct dma_slave_config *config);
> @@ -1194,6 +1201,34 @@ static inline struct dma_async_tx_descriptor *dmaengine_prep_interleaved_dma(
>  	return chan->device->device_prep_interleaved_dma(chan, xt, flags);
>  }
>  
> +/**
> + * dmaengine_prep_dma_sg() - Prepare a memory-to-memory scatter-gather DMA descriptor.
> + * @chan: The channel to be used for this descriptor
> + * @dst_sg: Destination scatter list
> + * @dst_nents: Number of entries in destination scatter list
> + * @src_sg: Source scatter list
> + * @src_nents: Number of entries in source scatter list
> + * @flags: DMA engine flags
> + *
> + * Prepares a DMA transaction that copies data from multiple source memory
> + * regions to multiple destination memory regions in a single DMA transaction.
> + * Entry i of the source list is paired with entry i of the destination list,
> + * so both lists must contain the same number of entries; the call returns
> + * NULL otherwise.
> + */
> +static inline struct dma_async_tx_descriptor *dmaengine_prep_dma_sg
> +		(struct dma_chan *chan,
> +		 struct scatterlist *dst_sg, unsigned int dst_nents,
> +		 struct scatterlist *src_sg, unsigned int src_nents,
> +		 unsigned long flags)
> +{
> +	if (!chan || !chan->device || !chan->device->device_prep_dma_sg)
> +		return NULL;

Can we check if  dst_nents is same as src_nents... or should we drop
src/dst and just say nents to imply both...

> +
> +	return chan->device->device_prep_dma_sg(chan, dst_sg, dst_nents,
> +			src_sg, src_nents, flags);
> +}
> +
>  /**
>   * dmaengine_prep_dma_memset() - Prepare a DMA memset descriptor.
>   * @chan: The channel to be used for this descriptor
> 
> -- 
> 2.34.1

-- 
~Vinod