| Message ID | a8efa43470bc5092b8727a93c9cf694c80e0c8c4.1564825752.git.lukas@wunner.de (mailing list archive) |
|---|---|
| State | Accepted |
| Headers | show |
| Series | Raspberry Pi SPI speedups | expand |
On 03-08-19, 12:10, Lukas Wunner wrote: > 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 Acked-by: Vinod Koul <vkoul@kernel.org> > > Tested-by: Nuno Sá <nuno.sa@analog.com> > Signed-off-by: Lukas Wunner <lukas@wunner.de> > Cc: Martin Sperl <kernel@martin.sperl.org> > Cc: Noralf Trønnes <noralf@tronnes.org> > Cc: Florian Kauer <florian.kauer@koalo.de> > --- > 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 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", > -- > 2.20.1
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",