Subject: [PATCH 3/5] dmaengine: Support for querying maximum trasnfer length (of an SG element)

From: [email-masked] (Vinod Koul)
Date: Thu, 21 Sep 2017 22:44:51 +0530

On Tue, Sep 12, 2017 at 01:44:22PM +0300, Peter Ujfalusi wrote:
> Certain DMA engines have limitation on the maximum size of a transfer they
> can support. This size limitation is per SG element or for period length in
> cyclic transfers.
> In TI's eDMA and sDMA this limitation is not really a length limit, but it
> is the number of bursts that we can support in one transfer.
> 
> With this callback the DMA drivers can provide hints to clients on how they
> should set up their buffers (sglist, cyclic buffer). Without this the
> clients must have open coded workarounds in place for each and every DMA
> engine they might be interfacing with to have correct length for the
> transfers.
> 
> Signed-off-by: Peter Ujfalusi <[email-masked]>
> ---
>  include/linux/dmaengine.h | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> index 8319101170fc..739824b94c1b 100644
> --- a/include/linux/dmaengine.h
> +++ b/include/linux/dmaengine.h
> @@ -705,6 +705,9 @@ struct dma_filter {
>   * @device_prep_dma_imm_data: DMA's 8 byte immediate data to the dst address
>   * @device_config: Pushes a new configuration to a channel, return 0 or an error
>   *	code
> + * @device_get_max_len: Get the maximum supported length in bytes of a slave
> + *	transfer based on the set dma_slave_config. The length limitation
> + *	applies to each SG element's length.
>   * @device_pause: Pauses any transfer happening on a channel. Returns
>   *	0 or an error code
>   * @device_resume: Resumes any transfer on a channel previously
> @@ -792,6 +795,8 @@ struct dma_device {
>  
>  	int (*device_config)(struct dma_chan *chan,
>  			     struct dma_slave_config *config);
> +	u32 (*device_get_max_len)(struct dma_chan *chan,
> +				  enum dma_transfer_direction dir);
>  	int (*device_pause)(struct dma_chan *chan);
>  	int (*device_resume)(struct dma_chan *chan);
>  	int (*device_terminate_all)(struct dma_chan *chan);
> @@ -812,6 +817,15 @@ static inline int dmaengine_slave_config(struct dma_chan *chan,
>  	return -ENOSYS;
>  }
>  
> +static inline u32 dmaengine_slave_get_max_len(struct dma_chan *chan,
> +					      enum dma_transfer_direction dir)
> +{
> +	if (chan->device->device_get_max_len)
> +		return chan->device->device_get_max_len(chan, dir);

not another callback :)

on a serious note, why shouldn't this be one more capability in
dma_slave_caps. looking at next patch it seems static

-- 
~Vinod