dmaengine Archive on lore.kernel.org
 help / color / Atom feed
From: Vinod Koul <vkoul@kernel.org>
To: Lukas Wunner <lukas@wunner.de>
Cc: Mark Brown <broonie@kernel.org>, Stefan Wahren <wahrenst@gmx.net>,
	linux-spi@vger.kernel.org, dmaengine@vger.kernel.org,
	linux-rpi-kernel@lists.infradead.org,
	bcm-kernel-feedback-list@broadcom.com,
	Eric Anholt <eric@anholt.net>, Nuno Sa <nuno.sa@analog.com>,
	Martin Sperl <kernel@martin.sperl.org>,
	Noralf Tronnes <noralf@tronnes.org>,
	Robert Jarzmik <robert.jarzmik@free.fr>,
	Florian Kauer <florian.kauer@koalo.de>,
	Florian Fainelli <f.fainelli@gmail.com>,
	Ray Jui <rjui@broadcom.com>,
	Scott Branden <sbranden@broadcom.com>
Subject: Re: [PATCH 09/10] dmaengine: bcm2835: Avoid accessing memory when copying zeroes
Date: Thu, 8 Aug 2019 18:01:52 +0530
Message-ID: <20190808123152.GW12733@vkoul-mobl.Dlink> (raw)
In-Reply-To: <a8efa43470bc5092b8727a93c9cf694c80e0c8c4.1564825752.git.lukas@wunner.de>

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

-- 
~Vinod

  reply index

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-03 10:10 [PATCH 00/10] Raspberry Pi SPI speedups Lukas Wunner
2019-08-03 10:10 ` [PATCH 01/10] dmaengine: bcm2835: Allow reusable descriptors Lukas Wunner
2019-08-08 12:30   ` Vinod Koul
2019-08-03 10:10 ` [PATCH 04/10] spi: bcm2835: Drop dma_pending flag Lukas Wunner
2019-08-03 10:10 ` [PATCH 05/10] spi: bcm2835: Work around DONE bit erratum Lukas Wunner
2019-08-11 19:45   ` Stefan Wahren
2019-08-11 19:57     ` Lukas Wunner
2019-08-11 20:29       ` Eric Anholt
2019-08-19 19:20       ` Stefan Wahren
2019-08-03 10:10 ` [PATCH 09/10] dmaengine: bcm2835: Avoid accessing memory when copying zeroes Lukas Wunner
2019-08-08 12:31   ` Vinod Koul [this message]
2019-08-03 10:10 ` [PATCH 07/10] spi: bcm2835: Speed up TX-only DMA transfers by clearing RX FIFO Lukas Wunner
2019-08-03 10:10 ` [PATCH 06/10] spi: bcm2835: Cache CS register value for ->prepare_message() Lukas Wunner
2019-08-03 10:10 ` [PATCH 02/10] dmaengine: bcm2835: Allow cyclic transactions without interrupt Lukas Wunner
2019-08-08 12:30   ` Vinod Koul
2019-08-03 10:10 ` [PATCH 08/10] dmaengine: bcm2835: Document struct bcm2835_dmadev Lukas Wunner
2019-08-08 12:31   ` Vinod Koul
2019-08-03 10:10 ` [PATCH 10/10] spi: bcm2835: Speed up RX-only DMA transfers by zero-filling TX FIFO Lukas Wunner
2019-08-03 10:10 ` [PATCH 03/10] spi: Guarantee cacheline alignment of driver-private data Lukas Wunner
2019-09-10 11:29   ` Mark Brown
2019-08-03 16:01 ` [PATCH 00/10] Raspberry Pi SPI speedups Noralf Trønnes
2019-08-11 19:50 ` Stefan Wahren
2019-08-11 19:52   ` Lukas Wunner
2019-08-19 19:22 ` Stefan Wahren
2019-08-21 15:21 ` kernel
2019-08-24 10:33 ` Lukas Wunner
2019-09-07  9:06 ` Lukas Wunner
2019-09-09 16:56   ` Mark Brown
2019-09-10 11:21   ` Mark Brown

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190808123152.GW12733@vkoul-mobl.Dlink \
    --to=vkoul@kernel.org \
    --cc=bcm-kernel-feedback-list@broadcom.com \
    --cc=broonie@kernel.org \
    --cc=dmaengine@vger.kernel.org \
    --cc=eric@anholt.net \
    --cc=f.fainelli@gmail.com \
    --cc=florian.kauer@koalo.de \
    --cc=kernel@martin.sperl.org \
    --cc=linux-rpi-kernel@lists.infradead.org \
    --cc=linux-spi@vger.kernel.org \
    --cc=lukas@wunner.de \
    --cc=noralf@tronnes.org \
    --cc=nuno.sa@analog.com \
    --cc=rjui@broadcom.com \
    --cc=robert.jarzmik@free.fr \
    --cc=sbranden@broadcom.com \
    --cc=wahrenst@gmx.net \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

dmaengine Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/dmaengine/0 dmaengine/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 dmaengine dmaengine/ https://lore.kernel.org/dmaengine \
		dmaengine@vger.kernel.org dmaengine@archiver.kernel.org
	public-inbox-index dmaengine

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.dmaengine


AGPL code for this site: git clone https://public-inbox.org/ public-inbox