Re: [RFC/PATCH] media: omap3isp: Set maximum DMA segment size

From: Laurent Pinchart
Date: Mon Nov 09 2015 - 09:18:44 EST


Hello everybody,

Ping ?

On Tuesday 13 October 2015 16:18:36 Laurent Pinchart wrote:
> The maximum DMA segment size controls the IOMMU mapping granularity. Its
> default value is 64kB, resulting in potentially non-contiguous IOMMU
> mappings. Configure it to 4GB to ensure that buffers get mapped
> contiguously.
>
> Signed-off-by: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
> ---
> drivers/media/platform/omap3isp/isp.c | 4 ++++
> drivers/media/platform/omap3isp/isp.h | 1 +
> 2 files changed, 5 insertions(+)
>
> I'm posting this as an RFC because I'm not happy with the patch, even if it
> gets the job done.
>
> On ARM the maximum DMA segment size is used when creating IOMMU mappings. As
> a large number of devices require contiguous memory buffers (this is a very
> common requirement for video-related embedded devices) the default 64kB
> value doesn't work.
>
> I haven't investigated the history behind this API in details but I have a
> feeling something is not quite right. We force most drivers to explicitly
> set the maximum segment size from a default that seems valid for specific
> use cases only. Furthermore, as the DMA parameters are not stored directly
> in struct device this require allocation of external memory for which we
> have no proper management rule, making automatic handling of the DMA
> parameters in frameworks or helper functions cumbersome (for a discussion
> on this topic see http://www.spinics.net/lists/linux-media/msg92467.html
> and http://lists.infradead.org/pipermail/linux-arm-kernel/2014-> November/305913.html).
>
> Is it time to fix this mess ?
>
> diff --git a/drivers/media/platform/omap3isp/isp.c
> b/drivers/media/platform/omap3isp/isp.c index 17430a6ed85a..ebf7dc76e94d
> 100644
> --- a/drivers/media/platform/omap3isp/isp.c
> +++ b/drivers/media/platform/omap3isp/isp.c
> @@ -2444,6 +2444,10 @@ static int isp_probe(struct platform_device *pdev)
> if (ret)
> goto error;
>
> + isp->dev->dma_parms = &isp->dma_parms;
> + dma_set_max_seg_size(isp->dev, DMA_BIT_MASK(32));
> + dma_set_seg_boundary(isp->dev, 0xffffffff);
> +
> platform_set_drvdata(pdev, isp);
>
> /* Regulators */
> diff --git a/drivers/media/platform/omap3isp/isp.h
> b/drivers/media/platform/omap3isp/isp.h index e579943175c4..4b2231cf01be
> 100644
> --- a/drivers/media/platform/omap3isp/isp.h
> +++ b/drivers/media/platform/omap3isp/isp.h
> @@ -193,6 +193,7 @@ struct isp_device {
> u32 syscon_offset;
> u32 phy_type;
>
> + struct device_dma_parameters dma_parms;
> struct dma_iommu_mapping *mapping;
>
> /* ISP Obj */

--
Regards,

Laurent Pinchart

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