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

From: Marek Szyprowski
Date: Tue Nov 17 2015 - 07:15:46 EST

Hi Laurent,

I really have no idea how this problem should be addressed. I've already proposed
to handle it in videobuf2-dc, but this has been rejected. Maybe the only way will
be to add dma_set_max_seg_size(isp->dev, DMA_BIT_MASK(32)) to every v4l2 driver,
which use videobuf2-dc and add a check in videobuf2-dc.

Best regards
Marek Szyprowski, PhD
Samsung R&D Institute Poland

On 2015-11-09 15:18, Laurent Pinchart wrote:
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

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
and> 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
--- 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
--- 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 */

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