On Mon, Jul 22, 2013 at 08:10:06PM +0200, Sebastian Andrzej Siewior wrote:
> In USB RX path it is possible that the we receive less bytes than
> requested. Take the following example:
> The driver for USB-to-UART submits an URB with 256 bytes in size and the
> dmaengine driver in turn programs a transfer of 256 bytes. The transfer
> is programmed and the dma engine waits for the data to arrive. Once data
> is sent on the UART the dma engine begins to move data. If there was
> only one data byte in the USB packet received then the DMA engine will
> only move one byte due to USB restrictions / rules. The real size of the
> transfer has to be notified to the user / caller so he set this to the
> caller.
> This patch adds the transfered member to the dma_async_tx_descriptor
> where the caller can obtain the final size.
>
> Cc: Vinod Koul <vinod.koul@xxxxxxxxx>
> Cc: Dan Williams <djbw@xxxxxx>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
> include/linux/dmaengine.h | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> index cb286b1..c3a4635 100644
> --- a/include/linux/dmaengine.h
> +++ b/include/linux/dmaengine.h
> @@ -403,6 +403,8 @@ typedef void (*dma_async_tx_callback)(void *dma_async_param);
> * @tx_submit: set the prepared descriptor(s) to be executed by the engine
> * @callback: routine to call after this operation is complete
> * @callback_param: general parameter to pass to the callback routine
> + * @transfered: number of bytes that were really transfered in case the channel
> + * transfered less than requested.
> * ---async_tx api specific fields---
> * @next: at completion submit this descriptor
> * @parent: pointer to the next level up in the dependency chain
> @@ -416,6 +418,7 @@ struct dma_async_tx_descriptor {
> dma_cookie_t (*tx_submit)(struct dma_async_tx_descriptor *tx);
> dma_async_tx_callback callback;
> void *callback_param;
> + unsigned int transfered;
I think this should be doable with residue as well. You can return DMA_SUCCESS
for status and fill in what is NOT transfered indicating that txn completed but
lesser number of bytes are valid
~Vinod
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