linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Marek Szyprowski <m.szyprowski@samsung.com>
To: Shuah Khan <shuahkh@osg.samsung.com>,
	linux@armlinux.org.uk, gregkh@linuxfoundation.org,
	pawel@osciak.com, kyungmin.park@samsung.com, mchehab@kernel.org
Cc: will.deacon@arm.com, Robin.Murphy@arm.com, jroedel@suse.de,
	bart.vanassche@sandisk.com, gregory.clement@free-electrons.com,
	acourbot@nvidia.com, festevam@gmail.com, krzk@kernel.org,
	niklas.soderlund+renesas@ragnatech.se, sricharan@codeaurora.org,
	dledford@redhat.com, vinod.koul@intel.com,
	andrew.smirnov@gmail.com, mauricfo@linux.vnet.ibm.com,
	alexander.h.duyck@intel.com, sagi@grimberg.me,
	ming.l@ssi.samsung.com, martin.petersen@oracle.com,
	javier@dowhile0.org, javier@osg.samsung.com,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-media@vger.kernel.org
Subject: Re: [PATCH] arm: dma: fix sharing of coherent DMA memory without struct page
Date: Fri, 14 Apr 2017 09:56:07 +0200	[thread overview]
Message-ID: <6d0c3e3c-8d1b-89bb-1392-6ffc7d8073c1@samsung.com> (raw)
In-Reply-To: <3afd77e5-2a98-42fd-b5c9-cbf4c32baa4f@osg.samsung.com>

Hi Shuah,

On 2017-04-11 00:50, Shuah Khan wrote:
> On 04/06/2017 06:01 AM, Marek Szyprowski wrote:
>> On 2017-04-05 18:02, Shuah Khan wrote:
>>> When coherent DMA memory without struct page is shared, importer
>>> fails to find the page and runs into kernel page fault when it
>>> tries to dmabuf_ops_attach/map_sg/map_page the invalid page found
>>> in the sg_table. Please see www.spinics.net/lists/stable/msg164204.html
>>> for more information on this problem.
>>>
>>> This solution allows coherent DMA memory without struct page to be
>>> shared by providing a way for the exporter to tag the DMA buffer as
>>> a special buffer without struct page association and passing the
>>> information in sg_table to the importer. This information is used
>>> in attach/map_sg to avoid cleaning D-cache and mapping.
>>>
>>> The details of the change are:
>>>
>>> Framework:
>>> - Add a new dma_attrs field to struct scatterlist.
>>> - Add a new DMA_ATTR_DEV_COHERENT_NOPAGE attribute to clearly identify
>>>     Coherent memory without struct page.
>>> - Add a new dma_check_dev_coherent() interface to check if memory is
>>>     the device coherent area. There is no way to tell where the memory
>>>     returned by dma_alloc_attrs() came from.
>>>
>>> Exporter logic:
>>> - Add logic to vb2_dc_alloc() to call dma_check_dev_coherent() and set
>>>     DMA_ATTR_DEV_COHERENT_NOPAGE based the results of the check. This is
>>>     done in the exporter context.
>>> - Add logic to arm_dma_get_sgtable() to identify memory without struct
>>>     page using DMA_ATTR_DEV_COHERENT_NOPAGE attribute. If this attr is
>>>     set, arm_dma_get_sgtable() will set page as the cpu_addr and update
>>>     dma_address and dma_attrs fields in struct scatterlist for this sgl.
>>>     This is done in exporter context when buffer is exported. With this
>>>     Note: This change is made on top of Russell King's patch that added
>>>     !pfn_valid(pfn) check to arm_dma_get_sgtable() to error out on invalid
>>>     pages. Coherent memory without struct page will trigger this error.
>>>
>>> Importer logic:
>>> - Add logic to vb2_dc_dmabuf_ops_attach() to identify memory without
>>>     struct page using DMA_ATTR_DEV_COHERENT_NOPAGE attribute when it copies
>>>     the sg_table from the exporter. It will copy dma_attrs and dma_address
>>>     fields. With this logic, dmabuf_ops_attach will no longer trip on an
>>>     invalid page.
>>> - Add logic to arm_dma_map_sg() to avoid mapping the page when sg_table
>>>     has DMA_ATTR_DEV_COHERENT_NOPAGE buffer.
>>> - Add logic to arm_dma_unmap_sg() to do nothing for sg entries with
>>>     DMA_ATTR_DEV_COHERENT_NOPAGE attribute.
>>>
>>> Without this change the following use-case that runs into kernel
>>> pagefault when importer tries to attach the exported buffer.
>>>
>>> With this change it works: (what a relief after watching pagefaults for
>>> weeks!!)
>>>
>>> gst-launch-1.0 filesrc location=~/GH3_MOV_HD.mp4 ! qtdemux ! h264parse ! v4l2video4dec capture-io-mode=dmabuf ! v4l2video7convert output-io-mode=dmabuf-import ! kmssink force-modesetting=true
>>>
>>> I am sending RFC patch to get feedback on the approach and see if I missed
>>> anything.
>> Frankly, once You decided to hack around dma-buf and issues with coherent,
>> carved out memory, it might be a bit better to find the ultimate solution
>> instead of the another hack. Please note that it will still not allow to
>> share a buffer allocated from carved-out memory and a device, which is
>> behind IOMMU.
> With your patch s5p-mfc patch series does address the problem for this
> use-case for 4.12 onwards. However I am still concerned about prior
> release and this pagefault is bad.

Right. It should simply fail with error code instead of pagefault.

> Invalid page test partially solves the problem. Would it helpful to
> at least prevent the pagfault with a definitive test. Please see my
> response to Russell. Let me know your thoughts on that.
>
>> I thought a bit about this and the current shape of dma-buf code.
>>
>> IMHO the proper way of solving all those issues would be to replace
>> dma-buf internal representation of the memory from struct scatter_list
>> to pfn array. This would really solve the problem of buffers which
>> cannot be properly represented by scatter lists/struct pages and would
>> even allow sharing buffers between all kinds of devices. Scatter-lists
>> are also quite over-engineered structures to represent a single buffer
>> (pfn array is a bit more compact representation). Also there is a lots
>> of buggy code which use scatter-list in a bit creative way (like
>> assuming that each page maps to a single scatter list entry for
>> example). The only missing piece, required for such change would be
>> extending DMA-mapping with dma_map_pfn() interface.
> I agree with you on scatterlists being clumsy. Changing over to pfn array
> could simplify things. I am exploring a slightly different option that
> might not require too many changes. I will respond with concrete ideas
> later on this week.

It looks that a similar issue is being worked on, see the following thread:
https://lkml.org/lkml/2017/4/13/710

>> This would be however quite large task, especially taking into account
>> all current users of DMA-buf framework...
> Yeah it will be a large task.

Maybe once scatterlist are switched to pfns, changing dmabuf internal
memory representation to pfn array might be much easier.

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

  reply	other threads:[~2017-04-14  7:56 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CGME20170405160251epcas4p14cc5d5f6064c84b133b9e280ac987a93@epcas4p1.samsung.com>
2017-04-05 16:02 ` Shuah Khan
2017-04-05 23:14   ` Russell King - ARM Linux
2017-04-10 14:52     ` Shuah Khan
2017-04-06  4:11   ` kbuild test robot
2017-04-06 12:01   ` Marek Szyprowski
2017-04-10 22:50     ` Shuah Khan
2017-04-14  7:56       ` Marek Szyprowski [this message]
2017-04-14  9:46         ` Russell King - ARM Linux
2017-04-17  1:10           ` Shuah Khan
2017-04-17 10:22             ` Russell King - ARM Linux
2017-04-19 23:38           ` Shuah Khan

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=6d0c3e3c-8d1b-89bb-1392-6ffc7d8073c1@samsung.com \
    --to=m.szyprowski@samsung.com \
    --cc=Robin.Murphy@arm.com \
    --cc=acourbot@nvidia.com \
    --cc=alexander.h.duyck@intel.com \
    --cc=andrew.smirnov@gmail.com \
    --cc=bart.vanassche@sandisk.com \
    --cc=dledford@redhat.com \
    --cc=festevam@gmail.com \
    --cc=gregkh@linuxfoundation.org \
    --cc=gregory.clement@free-electrons.com \
    --cc=javier@dowhile0.org \
    --cc=javier@osg.samsung.com \
    --cc=jroedel@suse.de \
    --cc=krzk@kernel.org \
    --cc=kyungmin.park@samsung.com \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-media@vger.kernel.org \
    --cc=linux@armlinux.org.uk \
    --cc=martin.petersen@oracle.com \
    --cc=mauricfo@linux.vnet.ibm.com \
    --cc=mchehab@kernel.org \
    --cc=ming.l@ssi.samsung.com \
    --cc=niklas.soderlund+renesas@ragnatech.se \
    --cc=pawel@osciak.com \
    --cc=sagi@grimberg.me \
    --cc=shuahkh@osg.samsung.com \
    --cc=sricharan@codeaurora.org \
    --cc=vinod.koul@intel.com \
    --cc=will.deacon@arm.com \
    --subject='Re: [PATCH] arm: dma: fix sharing of coherent DMA memory without struct page' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).