From d8dbcb77c74a68defb408fb0662cc5f989f8048a Mon Sep 17 00:00:00 2001 From: Lukas Wunner Date: Sat, 3 Aug 2019 12:10:00 +0200 Subject: [PATCH] dmaengine: bcm2835: Avoid accessing memory when copying zeroes MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Tested-by: Noralf Trønnes Signed-off-by: Lukas Wunner Acked-by: Vinod Koul Acked-by: Stefan Wahren Acked-by: Martin Sperl Cc: Florian Kauer --- drivers/dma/bcm2835-dma.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/drivers/dma/bcm2835-dma.c b/drivers/dma/bcm2835-dma.c index 6e05e888c5ac56..090b02f4707cd5 100644 --- a/drivers/dma/bcm2835-dma.c +++ b/drivers/dma/bcm2835-dma.c @@ -58,11 +58,14 @@ struct bcm2835_dma_cfg_data { * @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; const struct bcm2835_dma_cfg_data *cfg_data; }; @@ -925,6 +928,7 @@ 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; @@ -975,6 +979,10 @@ 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 */ @@ -1080,6 +1088,9 @@ 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); } int bcm2838_dma40_memcpy_init(void) @@ -1224,6 +1235,14 @@ 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; + } + of_id = of_match_node(bcm2835_dma_of_match, pdev->dev.of_node); if (!of_id) { dev_err(&pdev->dev, "Failed to match compatible string\n");