Re: [PATCH] media: vb2-dma-contig: configure DMA max segment size properly

From: Hans Verkuil <hverkuil@xs4all.nl>
To: Marek Szyprowski <m.szyprowski@samsung.com>,
	linux-media@vger.kernel.org, linux-samsung-soc@vger.kernel.org
Cc: Sylwester Nawrocki <s.nawrocki@samsung.com>,
	Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	Krzysztof Kozlowski <k.kozlowski@samsung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
	Hans Verkuil <hans.verkuil@cisco.com>
Subject: Re: [PATCH] media: vb2-dma-contig: configure DMA max segment size properly
Date: Thu, 28 Apr 2016 14:31:21 +0200	[thread overview]
Message-ID: <57220299.3000807@xs4all.nl> (raw)
In-Reply-To: <1461845540-26454-1-git-send-email-m.szyprowski@samsung.com>

Hi Marek,

Looks good, just a typo and a large sprinkling of extra 'the' in the text :-)

On 04/28/2016 02:12 PM, Marek Szyprowski wrote:
> This patch lets vb2-dma-contig memory allocator to configure DMA max
> segment size properly for the client device. Setting it is needed to let
> DMA-mapping subsystem to create a single, contiguous mapping in DMA
> address space. This is essential for all devices, which use dma-contig
> videobuf2 memory allocator and shared buffers (in USERPTR or DMAbuf modes
> of operations).
> 
> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> ---
> Hello,
> 
> This patch is a follow-up of my previous attempts to let Exynos
> multimedia devices to work properly with shared buffers when IOMMU is
> enabled:
> 1. https://www.mail-archive.com/linux-media@vger.kernel.org/msg96946.html
> 2. http://thread.gmane.org/gmane.linux.drivers.video-input-infrastructure/97316
> 3. https://patchwork.linuxtv.org/patch/30870/
> 
> As sugested by Hans, configuring DMA max segment size should be done by
> videobuf2-dma-contig module instead of requiring all device drivers to
> do it on their own.
> 
> Here is some backgroud why this is done in videobuf2-dc not in the
> respective generic bus code:
> http://lists.infradead.org/pipermail/linux-arm-kernel/2014-November/305913.html
> 
> Best regards,
> Marek Szyprowski
> ---
>  drivers/media/v4l2-core/videobuf2-dma-contig.c | 39 ++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/drivers/media/v4l2-core/videobuf2-dma-contig.c b/drivers/media/v4l2-core/videobuf2-dma-contig.c
> index 461ae55eaa98..871e370f8e88 100644
> --- a/drivers/media/v4l2-core/videobuf2-dma-contig.c
> +++ b/drivers/media/v4l2-core/videobuf2-dma-contig.c
> @@ -443,6 +443,35 @@ static void vb2_dc_put_userptr(void *buf_priv)
>  }
>  
>  /*
> + * To allow mapping scatter-list into signle chunk in DMA address space,

the scatter-list
signle -> a single
the DMA address space

> + * device is required to have DMA max segment size parameter set to value

the device
the DMA max segment
set to a value

> + * larger than the buffer size. Otherwise, DMA-mapping subsystem will

the DMA-mapping

> + * split the mapping into max segment size chunks.
> + * This function increases DMA max segment size parameter to let

the MDA max

> + * DMA-mapping to map a buffer as a single chunk in DMA address space.

the DMA-mapping map a buffer

> + * This code assumes that the DMA-mapping subsystem will merge all
> + * scatter-list segments if this is really possible (for example when
> + * IOMMU is available and enabled).

an IOMMU

> + * Ideally, this parameter should be set by generic bus code, but it is

the generic bus code

> + * left with the default 64KiB value due to some historical limitations
> + * in other subsystems (like limited USB host drivers) and there is no
> + * good place to set it to the proper value. It is done here to avoid
> + * fixing all vb2-dc client drivers.

all the vb2-dc

> + */
> +static int vb2_dc_set_max_seg_size(struct device *dev, unsigned int size)
> +{
> +	if (!dev->dma_parms) {
> +		dev->dma_parms = kzalloc(sizeof(dev->dma_parms), GFP_KERNEL);
> +		if (!dev->dma_parms)
> +			return -ENOMEM;
> +	}
> +	if (dma_get_max_seg_size(dev) < size)
> +		return dma_set_max_seg_size(dev, size);
> +
> +	return 0;
> +}
> +
> +/*
>   * For some kind of reserved memory there might be no struct page available,
>   * so all that can be done to support such 'pages' is to try to convert
>   * pfn to dma address or at the last resort just assume that
> @@ -499,6 +528,10 @@ static void *vb2_dc_get_userptr(struct device *dev, unsigned long vaddr,
>  		return ERR_PTR(-EINVAL);
>  	}
>  
> +	ret = vb2_dc_set_max_seg_size(dev, PAGE_ALIGN(size + PAGE_SIZE));
> +	if (!ret)
> +		return ERR_PTR(ret);
> +
>  	buf = kzalloc(sizeof *buf, GFP_KERNEL);
>  	if (!buf)
>  		return ERR_PTR(-ENOMEM);
> @@ -675,10 +708,15 @@ static void *vb2_dc_attach_dmabuf(struct device *dev, struct dma_buf *dbuf,
>  {
>  	struct vb2_dc_buf *buf;
>  	struct dma_buf_attachment *dba;
> +	int ret;
>  
>  	if (dbuf->size < size)
>  		return ERR_PTR(-EFAULT);
>  
> +	ret = vb2_dc_set_max_seg_size(dev, PAGE_ALIGN(size));
> +	if (!ret)
> +		return ERR_PTR(ret);
> +
>  	buf = kzalloc(sizeof(*buf), GFP_KERNEL);
>  	if (!buf)
>  		return ERR_PTR(-ENOMEM);
> @@ -722,6 +760,7 @@ const struct vb2_mem_ops vb2_dma_contig_memops = {
>  };
>  EXPORT_SYMBOL_GPL(vb2_dma_contig_memops);
>  
> +

Spurious newline. Intended or a mistake?

>  MODULE_DESCRIPTION("DMA-contig memory handling routines for videobuf2");
>  MODULE_AUTHOR("Pawel Osciak <pawel@osciak.com>");
>  MODULE_LICENSE("GPL");
> 

Regards,

	Hans