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    SubjectRe: [PATCH v3] staging: android: ion: Add implementation of dma_buf_vmap and dma_buf_vunmap
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    On 12/17/18 20:42, Liam Mark wrote:
    > On Sun, 16 Dec 2018, Alexey Skidanov wrote:
    >
    >>
    >>
    >> On 12/16/18 7:20 AM, Liam Mark wrote:
    >>> On Tue, 6 Feb 2018, Alexey Skidanov wrote:
    >>>
    >>>>
    >>>>
    >>>> On 02/07/2018 01:56 AM, Laura Abbott wrote:
    >>>>> On 01/31/2018 10:10 PM, Alexey Skidanov wrote:
    >>>>>>
    >>>>>> On 01/31/2018 03:00 PM, Greg KH wrote:
    >>>>>>> On Wed, Jan 31, 2018 at 02:03:42PM +0200, Alexey Skidanov wrote:
    >>>>>>>> Any driver may access shared buffers, created by ion, using
    >>>>>>>> dma_buf_vmap and
    >>>>>>>> dma_buf_vunmap dma-buf API that maps/unmaps previosuly allocated
    >>>>>>>> buffers into
    >>>>>>>> the kernel virtual address space. The implementation of these API is
    >>>>>>>> missing in
    >>>>>>>> the current ion implementation.
    >>>>>>>>
    >>>>>>>> Signed-off-by: Alexey Skidanov <alexey.skidanov@intel.com>
    >>>>>>>> ---
    >>>>>>>
    >>>>>>> No review from any other Intel developers? :(
    >>>>>> Will add.
    >>>>>>>
    >>>>>>> Anyway, what in-tree driver needs access to these functions?
    >>>>>> I'm not sure that there are the in-tree drivers using these functions
    >>>>>> and ion as> buffer exporter because they are not implemented in ion :)
    >>>>>> But there are some in-tre> drivers using these APIs (gpu drivers) with
    >>>>>> other buffer exporters.
    >>>>>
    >>>>> It's still not clear why you need to implement these APIs.
    >>>> How the importing kernel module may access the content of the buffer? :)
    >>>> With the current ion implementation it's only possible by dma_buf_kmap,
    >>>> mapping one page at a time. For pretty large buffers, it might have some
    >>>> performance impact.
    >>>> (Probably, the page by page mapping is the only way to access large
    >>>> buffers on 32 bit systems, where the vmalloc range is very small. By the
    >>>> way, the current ion dma_map_kmap doesn't really map only 1 page at a
    >>>> time - it uses the result of vmap() that might fail on 32 bit systems.)
    >>>>
    >>>>> Are you planning to use Ion with GPU drivers? I'm especially
    >>>>> interested in this if you have a non-Android use case.
    >>>> Yes, my use case is the non-Android one. But not with GPU drivers.
    >>>>>
    >>>>> Thanks,
    >>>>> Laura
    >>>>
    >>>> Thanks,
    >>>> Alexey
    >>>
    >>> I was wondering if we could re-open the discussion on adding support to
    >>> ION for dma_buf_vmap.
    >>> It seems like the patch was not taken as the reviewers wanted more
    >>> evidence of an upstream use case.
    >>>
    >>> Here would be my upstream usage argument for including dma_buf_vmap
    >>> support in ION.
    >>>
    >>> Currently all calls to ion_dma_buf_begin_cpu_access result in the creation
    >>> of a kernel mapping for the buffer, unfortunately the resulting call to
    >>> alloc_vmap_area can be quite expensive and this has caused a performance
    >>> regression for certain clients when they have moved to the new version of
    >>> ION.
    >>>
    >>> The kernel mapping is not actually needed in ion_dma_buf_begin_cpu_access,
    >>> and generally isn't needed by clients. So if we remove the creation of the
    >>> kernel mapping in ion_dma_buf_begin_cpu_access and only create it when
    >>> needed we can speed up the calls to ion_dma_buf_begin_cpu_access.
    >>>
    >>> An additional benefit of removing the creation of kernel mappings from
    >>> ion_dma_buf_begin_cpu_access is that it makes the ION code more secure.
    >>> Currently a malicious client could call the DMA_BUF_IOCTL_SYNC IOCTL with
    >>> flags DMA_BUF_SYNC_END multiple times to cause the ION buffer kmap_cnt to
    >>> go negative which could lead to undesired behavior.
    >>>
    >>> One disadvantage of the above change is that a kernel mapping is not
    >>> already created when a client calls dma_buf_kmap. So the following
    >>> dma_buf_kmap contract can't be satisfied.
    >>>
    >>> /**
    >>> * dma_buf_kmap - Map a page of the buffer object into kernel address
    >>> space. The
    >>> * same restrictions as for kmap and friends apply.
    >>> * @dmabuf: [in] buffer to map page from.
    >>> * @page_num: [in] page in PAGE_SIZE units to map.
    >>> *
    >>> * This call must always succeed, any necessary preparations that might
    >>> fail
    >>> * need to be done in begin_cpu_access.
    >>> */
    >>>
    >>> But hopefully we can work around this by moving clients to dma_buf_vmap.
    >> I think the problem is with the contract. We can't ensure that the call
    >> is always succeeds regardless the implementation - any mapping might
    >> fail. Probably this is why *all* clients of dma_buf_kmap() check the
    >> return value (so it's safe to return NULL in case of failure).
    >>
    >
    > I think currently the call to dma_buf_kmap will always succeed since the
    > DMA-Buf contract requires that the client first successfully call
    > dma_buf_begin_cpu_access(), and if dma_buf_begin_cpu_access() succeeds
    > then dma_buf_kmap will succeed.
    >
    >> I would suggest to fix the contract and to keep the dma_buf_kmap()
    >> support in ION.
    >
    > I will leave it to the DMA-Buf maintainers as to whether they want to
    > change their contract.
    >
    > Liam
    >
    > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
    > a Linux Foundation Collaborative Project
    >

    Ok. We need the list of the clients using the ION in the mainline tree.

    Alexey

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