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

[laurent.pinchart@ideasonboard.com (Laurent Pinchart)]

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@ideasonboard.com>
> ---
>  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