Subject: Re: [RFC/PATCH] media: omap3isp: Set maximum DMA segment size
To: Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        linux-media@vger.kernel.org
References: <1444742316-27986-1-git-send-email-laurent.pinchart@ideasonboard.com>
 <1560214.1Y1q0qLZA4@avalon>
Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        iommu@lists.linux-foundation.org, robin.murphy@arm.com,
        Lars-Peter Clausen <lars@metafoo.de>, sakari.ailus@iki.fi,
        Shuah Khan <shuahkhan@gmail.com>
From: Marek Szyprowski <m.szyprowski@samsung.com>
Message-id: <564B1A4E.7040001@samsung.com>
Date: Tue, 17 Nov 2015 13:15:10 +0100
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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
>> 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 */

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