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From: Marek Szyprowski <m.szyprowski@samsung.com>
To: Christoph Hellwig <hch@lst.de>
Cc: "Niklas Söderlund" <niklas.soderlund+renesas@ragnatech.se>,
	"Pawel Osciak" <pawel@osciak.com>,
	linux-kernel@vger.kernel.org,
	"Kyungmin Park" <kyungmin.park@samsung.com>,
	"Russell King" <linux@armlinux.org.uk>,
	iommu@lists.linux-foundation.org,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
	linuxppc-dev@lists.ozlabs.org,
	linux-arm-kernel@lists.infradead.org,
	linux-media@vger.kernel.org
Subject: Re: [PATCH 3/3] videobuf2: replace a layering violation with dma_map_resource
Date: Fri, 18 Jan 2019 10:41:06 +0100	[thread overview]
Message-ID: <f4222474-0515-c0a2-ef5e-523b56869210@samsung.com> (raw)
In-Reply-To: <20190117172152.GA32292@lst.de>

Hi Christoph,

On 2019-01-17 18:21, Christoph Hellwig wrote:
> On Mon, Jan 14, 2019 at 01:42:26PM +0100, Marek Szyprowski wrote:
>> On 2019-01-11 19:17, Christoph Hellwig wrote:
>>> vb2_dc_get_userptr pokes into arm direct mapping details to get the
>>> resemblance of a dma address for a a physical address that does is
>>> not backed by a page struct.  Not only is this not portable to other
>>> architectures with dma direct mapping offsets, but also not to uses
>>> of IOMMUs of any kind.  Switch to the proper dma_map_resource /
>>> dma_unmap_resource interface instead.
>>>
>>> Signed-off-by: Christoph Hellwig <hch@lst.de>
>> There are checks for IOMMU presence in other places in vb2-dma-contig,
>> so it was used only when no IOMMU is available, but I agree that the
>> hacky code should be replaced by a generic code if possible.
>>
>> Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
>>
>> V4L2 pipeline works fine on older Exynos-based boards with CMA and IOMMU
>> disabled.
> Do you know if these rely on the offsets?  E.g. would they still work
> with the patch below applied on top.  That would keep the map_resource
> semantics as-is as solve the issue pointed out by Robin for now.

AFAIK that code was only used for sharing buffers between hardware
modules that are a part of the same SoC, usually implemented as platform
devices. AFAIR this never worked for devices on different buses. So far
I wasn't aware on ANY which would require an offset for the DMA access.

The first version of videobuf2-dc code even incorrectly used paddr
instead of dma_addr as a buffer 'address' returned to the client
drivers, because in case of those SoC this was exactly the same (see
commits 472af2b05bdefcaee7e754e22cbf131110017ad6 and
ba7fcb0c954921534707f08ebc4d8beeb2eb17e7).

> If not I can only think of a flag to bypass the offseting for now, but
> that would be pretty ugly.  Or go for the long-term solution of
> discovering the relationship between the two devices, as done by the
> PCIe P2P code..
>
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 8e0359b04957..25bd19974223 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -359,7 +359,7 @@ EXPORT_SYMBOL(dma_direct_map_sg);
>  dma_addr_t dma_direct_map_resource(struct device *dev, phys_addr_t paddr,
>  		size_t size, enum dma_data_direction dir, unsigned long attrs)
>  {
> -	dma_addr_t dma_addr = phys_to_dma(dev, paddr);
> +	dma_addr_t dma_addr = paddr;
>  
>  	if (unlikely(!dma_direct_possible(dev, dma_addr, size))) {
>  		report_addr(dev, dma_addr, size);
>
>
>
Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland


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  reply	other threads:[~2019-01-18  9:41 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CGME20190111181841epcas3p2d7c0bf8f5c11a9863e22ec1b12da6e1b@epcas3p2.samsung.com>
2019-01-11 18:17 ` fix a layering violation in videobuf2 and improve dma_map_resource Christoph Hellwig
2019-01-11 18:17   ` [PATCH 1/3] dma-mapping: remove the default map_resource implementation Christoph Hellwig
2019-01-14 13:12     ` Robin Murphy
2019-01-14 17:08       ` Christoph Hellwig
2019-01-11 18:17   ` [PATCH 2/3] dma-mapping: don't BUG when calling dma_map_resource on RAM Christoph Hellwig
2019-01-14 14:16     ` Robin Murphy
2019-01-11 18:17   ` [PATCH 3/3] videobuf2: replace a layering violation with dma_map_resource Christoph Hellwig
2019-01-11 19:54     ` Mauro Carvalho Chehab
2019-01-14 10:31       ` Christoph Hellwig
2019-01-14 11:04         ` Mauro Carvalho Chehab
2019-01-14 11:12           ` Christoph Hellwig
2019-01-14 12:42     ` Marek Szyprowski
2019-01-17 17:21       ` Christoph Hellwig
2019-01-18  9:41         ` Marek Szyprowski [this message]
2019-01-31 15:31           ` Christoph Hellwig
2019-01-14 11:43   ` fix a layering violation in videobuf2 and improve dma_map_resource Marek Szyprowski
2019-01-18 11:37 fix a layering violation in videobuf2 and improve dma_map_resource v2 Christoph Hellwig
2019-01-18 11:37 ` [PATCH 3/3] videobuf2: replace a layering violation with dma_map_resource Christoph Hellwig

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