Re: [PATCH RFC v4 07/15] spi: add offload TX/RX streaming APIs

["Nuno Sá" <noname.nuno@gmail.com>]

On Wed, 2024-10-23 at 15:59 -0500, David Lechner wrote:
> Most configuration of SPI offloads is handled opaquely using the offload
> pointer that is passed to the various offload functions. However, there
> are some offload features that need to be controlled on a per transfer
> basis.
> 
> This patch adds a flag field to struct spi_transfer to allow specifying
> such features. The first feature to be added is the ability to stream
> data to/from a hardware sink/source rather than using a tx or rx buffer.
> Additional flags can be added in the future as needed.
> 
> A flags field is also added to the offload struct for providers to
> indicate which flags are supported. This allows for generic checking of
> offload capabilities during __spi_validate() so that each offload
> provider doesn't have to implement their own validation.
> 
> As a first users of this streaming capability, getter functions are
> added to get a DMA channel that is directly connected to the offload.
> Peripheral drivers will use this to get a DMA channel and configure it
> to suit their needs.
> 
> Signed-off-by: David Lechner <dlechner@baylibre.com>
> ---
> 
> v4 changes:
> * DMA API's now automatically release DMA channels instead of leaving
>   it up to the caller.
> 
> v3 changes:
> * Added spi_offload_{tx,rx}_stream_get_dma_chan() functions.
> 
> v2 changes:
> * This is also split out from "spi: add core support for controllers with
>   offload capabilities".
> * In the previous version, we were using (void *)-1 as a sentinel value
>   that could be assigned, e.g. to rx_buf. But this was naive since there
>   is core code that would try to dereference this pointer. So instead,
>   we've added a new flags field to the spi_transfer structure for this
>   sort of thing. This also has the advantage of being able to be used in
>   the future for other arbitrary features.
> ---
>  drivers/spi/spi-offload.c       | 76 +++++++++++++++++++++++++++++++++++++++++
>  drivers/spi/spi.c               | 10 ++++++
>  include/linux/spi/spi-offload.h | 24 +++++++++++++
>  include/linux/spi/spi.h         |  3 ++
>  4 files changed, 113 insertions(+)
> 
> diff --git a/drivers/spi/spi-offload.c b/drivers/spi/spi-offload.c
> index 2a1f9587f27a..dd4cb3c2e985 100644
> --- a/drivers/spi/spi-offload.c
> +++ b/drivers/spi/spi-offload.c
> @@ -8,6 +8,7 @@
>  
>  #include <linux/cleanup.h>
>  #include <linux/device.h>
> +#include <linux/dmaengine.h>
>  #include <linux/export.h>
>  #include <linux/list.h>
>  #include <linux/mutex.h>
> @@ -282,6 +283,81 @@ void spi_offload_trigger_disable(struct spi_offload *offload,
>  }
>  EXPORT_SYMBOL_GPL(spi_offload_trigger_disable);
>  
> +static void spi_offload_release_dma_chan(void *chan)
> +{
> +	dma_release_channel(chan);
> +}
> +
> +/**
> + * spi_offload_tx_stream_request_dma_chan_info - Get the DMA channel info for the
> TX stream
> + * @spi: SPI device
> + * @id: Function ID if SPI device uses more than one offload or NULL.
> + *
> + * This is the DMA channel that will provide data to transfers that use the
> + * %SPI_OFFLOAD_XFER_TX_STREAM offload flag.
> + *
> + * The caller is responsible for calling spi_offload_free_dma_chan_info() on the
> + * returned pointer.

I guess the above does not make sense now. But I would still document (just to really
make it explicit) that the lifetime of the DMA channel is effectively being handed
over to the consumer.



- Nuno Sá