Re: [PATCH v3] media: vb2-dma-contig: configure DMA max segment size properly

[Hans Verkuil <hverkuil@xs4all.nl>]

On 05/02/16 12:59, 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
> 
> changelog:
> v3:
> - added FIXME note about possible memory leak
> 
> v2:
> - fixes typos and other language issues in the comments
> 
> v1: http://article.gmane.org/gmane.linux.kernel.samsung-soc/53690
> ---
>  drivers/media/v4l2-core/videobuf2-dma-contig.c | 45 ++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
> 
> diff --git a/drivers/media/v4l2-core/videobuf2-dma-contig.c b/drivers/media/v4l2-core/videobuf2-dma-contig.c
> index 461ae55eaa98..2ca7e798f394 100644
> --- a/drivers/media/v4l2-core/videobuf2-dma-contig.c
> +++ b/drivers/media/v4l2-core/videobuf2-dma-contig.c
> @@ -443,6 +443,42 @@ static void vb2_dc_put_userptr(void *buf_priv)
>  }
>  
>  /*
> + * To allow mapping the scatter-list into a single chunk in the DMA
> + * address space, the device is required to have the DMA max segment
> + * size parameter set to a value larger than the buffer size. Otherwise,
> + * the DMA-mapping subsystem will split the mapping into max segment
> + * size chunks. This function increases the DMA max segment size
> + * parameter to let DMA-mapping map a buffer as a single chunk in DMA
> + * address space.
> + * This code assumes that the DMA-mapping subsystem will merge all
> + * scatterlist segments if this is really possible (for example when
> + * an IOMMU is available and enabled).
> + * Ideally, this parameter should be set by the generic bus code, but it
> + * is left with the default 64KiB value due to historical litmiations in
> + * other subsystems (like limited USB host drivers) and there no good
> + * place to set it to the proper value. It is done here to avoid fixing
> + * all the vb2-dc client drivers.
> + *
> + * FIXME: the allocated dma_params structure is leaked because there
> + * is completely no way to determine when to free it (dma_params might have
> + * been also already allocated by the bus code). However in typical
> + * use cases this function will be called for platform devices, which are
> + * not how-plugged and exist all the time in the target system.

how-plugged, hmm. I'll change that to hot-plugged before merging :-)

	Hans

> + */
> +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 +535,10 @@ 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 +715,15 @@ 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);
>