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authorLukas Wunner <lukas@wunner.de>2019-09-11 12:15:30 +0200
committerMark Brown <broonie@kernel.org>2019-09-11 15:56:46 +0100
commitbf75703d0912da7b1af72f14392c805b758c3fbc (patch)
tree82fcb3f3b02e822551e02a6da940d8ab657a30ce /drivers/dma
parent571e31fa60b3697d5db26140e16d5c45c51c9815 (diff)
dmaengine: bcm2835: Avoid accessing memory when copying zeroes
The BCM2835 DMA controller is capable of synthesizing zeroes instead of copying them from a source address. The feature is enabled by setting the SRC_IGNORE bit in the Transfer Information field of a Control Block: "Do not perform source reads. In addition, destination writes will zero all the write strobes. This is used for fast cache fill operations." https://www.raspberrypi.org/app/uploads/2012/02/BCM2835-ARM-Peripherals.pdf The feature is only available on 8 of the 16 channels. The others are so-called "lite" channels with a limited feature set and performance. Enable the feature if a cyclic transaction copies from the zero page. This reduces traffic on the memory bus. A forthcoming use case is the BCM2835 SPI driver, which will cyclically copy from the zero page to the TX FIFO. The idea to use SRC_IGNORE was taken from an ancient GitHub conversation between Martin and Noralf: https://github.com/msperl/spi-bcm2835/issues/13#issuecomment-98180451 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/b2286c904408745192e4beb3de3c88f73e4a7210.1568187525.git.lukas@wunner.de Signed-off-by: Mark Brown <broonie@kernel.org>
Diffstat (limited to 'drivers/dma')
-rw-r--r--drivers/dma/bcm2835-dma.c19
1 files changed, 19 insertions, 0 deletions
diff --git a/drivers/dma/bcm2835-dma.c b/drivers/dma/bcm2835-dma.c
index 14358faf3bff..67100e4e1083 100644
--- a/drivers/dma/bcm2835-dma.c
+++ b/drivers/dma/bcm2835-dma.c
@@ -42,11 +42,14 @@
* @ddev: DMA device
* @base: base address of register map
* @dma_parms: DMA parameters (to convey 1 GByte max segment size to clients)
+ * @zero_page: bus address of zero page (to detect transactions copying from
+ * zero page and avoid accessing memory if so)
*/
struct bcm2835_dmadev {
struct dma_device ddev;
void __iomem *base;
struct device_dma_parameters dma_parms;
+ dma_addr_t zero_page;
};
struct bcm2835_dma_cb {
@@ -693,6 +696,7 @@ static struct dma_async_tx_descriptor *bcm2835_dma_prep_dma_cyclic(
size_t period_len, enum dma_transfer_direction direction,
unsigned long flags)
{
+ struct bcm2835_dmadev *od = to_bcm2835_dma_dev(chan->device);
struct bcm2835_chan *c = to_bcm2835_dma_chan(chan);
struct bcm2835_desc *d;
dma_addr_t src, dst;
@@ -743,6 +747,10 @@ static struct dma_async_tx_descriptor *bcm2835_dma_prep_dma_cyclic(
dst = c->cfg.dst_addr;
src = buf_addr;
info |= BCM2835_DMA_D_DREQ | BCM2835_DMA_S_INC;
+
+ /* non-lite channels can write zeroes w/o accessing memory */
+ if (buf_addr == od->zero_page && !c->is_lite_channel)
+ info |= BCM2835_DMA_S_IGNORE;
}
/* calculate number of frames */
@@ -845,6 +853,9 @@ static void bcm2835_dma_free(struct bcm2835_dmadev *od)
list_del(&c->vc.chan.device_node);
tasklet_kill(&c->vc.task);
}
+
+ dma_unmap_page_attrs(od->ddev.dev, od->zero_page, PAGE_SIZE,
+ DMA_TO_DEVICE, DMA_ATTR_SKIP_CPU_SYNC);
}
static const struct of_device_id bcm2835_dma_of_match[] = {
@@ -927,6 +938,14 @@ static int bcm2835_dma_probe(struct platform_device *pdev)
platform_set_drvdata(pdev, od);
+ od->zero_page = dma_map_page_attrs(od->ddev.dev, ZERO_PAGE(0), 0,
+ PAGE_SIZE, DMA_TO_DEVICE,
+ DMA_ATTR_SKIP_CPU_SYNC);
+ if (dma_mapping_error(od->ddev.dev, od->zero_page)) {
+ dev_err(&pdev->dev, "Failed to map zero page\n");
+ return -ENOMEM;
+ }
+
/* Request DMA channel mask from device tree */
if (of_property_read_u32(pdev->dev.of_node,
"brcm,dma-channel-mask",