On 03-08-19, 12:10, Lukas Wunner wrote:
> The BCM2835 DMA driver currently requests an interrupt from the
> controller regardless whether or not the client has passed in the
> DMA_PREP_INTERRUPT flag. This causes unnecessary overhead for cyclic
> transactions which do not need an interrupt after each period.
>
> We're about to add such a use case, namely cyclic clearing of the SPI
> controller's RX FIFO, so amend the DMA driver to request an interrupt
> only if DMA_PREP_INTERRUPT was passed in. Ignore the period_len for
> such transactions and set it to the buffer length to make the driver's
> calculations work.
Acked-by: Vinod Koul <vkoul@xxxxxxxxxx>
>
> Tested-by: Nuno Sá <nuno.sa@xxxxxxxxxx>
> Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx>
> Cc: Martin Sperl <kernel@xxxxxxxxxxxxxxxx>
> Cc: Florian Kauer <florian.kauer@xxxxxxxx>
> ---
> drivers/dma/bcm2835-dma.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/dma/bcm2835-dma.c b/drivers/dma/bcm2835-dma.c
> index 523c507ad69e..a65514fcb7f2 100644
> --- a/drivers/dma/bcm2835-dma.c
> +++ b/drivers/dma/bcm2835-dma.c
> @@ -691,7 +691,7 @@ static struct dma_async_tx_descriptor *bcm2835_dma_prep_dma_cyclic(
> struct bcm2835_desc *d;
> dma_addr_t src, dst;
> u32 info = BCM2835_DMA_WAIT_RESP;
> - u32 extra = BCM2835_DMA_INT_EN;
> + u32 extra = 0;
> size_t max_len = bcm2835_dma_max_frame_length(c);
> size_t frames;
>
> @@ -707,6 +707,11 @@ static struct dma_async_tx_descriptor *bcm2835_dma_prep_dma_cyclic(
> return NULL;
> }
>
> + if (flags & DMA_PREP_INTERRUPT)
> + extra |= BCM2835_DMA_INT_EN;
> + else
> + period_len = buf_len;
> +
> /*
> * warn if buf_len is not a multiple of period_len - this may leed
> * to unexpected latencies for interrupts and thus audiable clicks
> @@ -778,7 +783,10 @@ static int bcm2835_dma_terminate_all(struct dma_chan *chan)
>
> /* stop DMA activity */
> if (c->desc) {
> - vchan_terminate_vdesc(&c->desc->vd);
> + if (c->desc->vd.tx.flags & DMA_PREP_INTERRUPT)
> + vchan_terminate_vdesc(&c->desc->vd);
> + else
> + vchan_vdesc_fini(&c->desc->vd);
> c->desc = NULL;
> bcm2835_dma_abort(c);
> }
> --
> 2.20.1
--
~Vinod
