dmaengine: bcm2835: Allow reusable descriptors

The DMA engine API requires DMA drivers to explicitly allow that
descriptors are prepared once and reused multiple times. Only a
single driver makes use of this functionality so far (pxa_dma.c,
to speed up pxa_camera.c).

We're about to add another use case for reusable descriptors in
the BCM2835 SPI driver, so allow that in the BCM2835 DMA driver.
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>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Link: https://lore.kernel.org/r/bfc98a38225bbec4158440ad06cb9eee675e3e6f.1568187525.git.lukas@wunner.deSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/dma/bcm2835-dma.c

@@ -915,6 +915,7 @@ static int bcm2835_dma_probe(struct platform_device *pdev)
 	od->ddev.directions = BIT(DMA_DEV_TO_MEM) | BIT(DMA_MEM_TO_DEV) |
 			      BIT(DMA_MEM_TO_MEM);
 	od->ddev.residue_granularity = DMA_RESIDUE_GRANULARITY_BURST;
+	od->ddev.descriptor_reuse = true;
 	od->ddev.dev = &pdev->dev;
 	INIT_LIST_HEAD(&od->ddev.channels);