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

Vinod Koul
2017-09-21T17:19:32Z

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 <peter.ujfalusi@ti.com>
> ---
> 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