dmaengine: bcm2835: Allow cyclic transactions without interrupt

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.
Tested-by: Nuno Sá <nuno.sa@analog.com>
Tested-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Vinod Koul <vkoul@kernel.org>
Acked-by: Stefan Wahren <wahrenst@gmx.net>
Acked-by: Martin Sperl <kernel@martin.sperl.org>
Cc: Florian Kauer <florian.kauer@koalo.de>
Link: https://lore.kernel.org/r/73cf37be56eb4cbe6f696057c719f3a38cbaf26e.1568187525.git.lukas@wunner.deSigned-off-by: Mark Brown <broonie@kernel.org>

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);
 	}