diff options
| author | Lukas Wunner <lukas@wunner.de> | 2019-09-11 12:15:30 +0200 |
|---|---|---|
| committer | Mark Brown <broonie@kernel.org> | 2019-09-11 15:56:46 +0100 |
| commit | bf75703d0912da7b1af72f14392c805b758c3fbc (patch) | |
| tree | 82fcb3f3b02e822551e02a6da940d8ab657a30ce /drivers/dma | |
| parent | 571e31fa60b3697d5db26140e16d5c45c51c9815 (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.c | 19 |
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", |