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 <[email protected]>
> ---
> 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
Hi Laurent,
On 09/11/15 14:18, Laurent Pinchart wrote:
> Hello everybody,
>
> Ping ?
Apologies, I did start writing a response a while ago, but it ended up
getting subsumed into the bigger DMA API discussion thread.
> 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 <[email protected]>
>> ---
>> 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.
Per the initial patch (6b7b65105522), dma_parms was intended to expose
hardware limitations of scatter-gather-capable devices, to prevent DMA
API implementations from merging segments beyond a device's limits. Thus
the way 32-bit ARM (and seemingly noone else) is taking it as something
to apply to non-scatter-gather-capable devices in order to force the DMA
API implementation to merge segments seems very questionable.
There's nothing in the streaming DMA API which gives any guarantee of a
contiguous mapping, so it's the incorrect assumption that it does which
needs fixing. Whether we rework the callers or the API itself is the
open question there, I think.
>> 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 ?
I agree that it would certainly be preferable to tackle the underlying
issue instead of adding more point hacks to further entrench
non-portable code into the kernel. In terms of modifying the API, the
most reasonable idea which comes to mind would be a DMA attribute, and
on closer inspection, I see that DMA_ATTR_FORCE_CONTIGUOUS is already a
thing - perhaps we should weigh up whether coherent and streaming DMA
could overload the same attribute with subtly different meanings, or
whether we'd want e.g. DMA_ATTR_FORCE_PHYS_CONTIGUOUS and
DMA_ATTR_FORCE_DMA_CONTIGUOUS to coexist.
Robin.
>> 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);
Whatever happens, 002edb6f6f2a should now make this line redundant :D
>> +
>> 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 */
>
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 <[email protected]>
>> ---
>> 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 */