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    SubjectRE: [Xen-devel] [RFC] virtio_ring: check dma_mem for xen_domain
    On Fri, 25 Jan 2019, Peng Fan wrote:
    > > On Wed, Jan 23, 2019 at 01:04:33PM -0800, Stefano Stabellini wrote:
    > > > If vring_use_dma_api is actually supposed to return true when
    > > > dma_dev->dma_mem is set, then both Peng's patch and the patch I wrote
    > > > are not fixing the real issue here.
    > > >
    > > > I don't know enough about remoteproc to know where the problem
    > > > actually lies though.
    > >
    > > The problem is the following:
    > >
    > > Devices can declare a specific memory region that they want to use when the
    > > driver calls dma_alloc_coherent for the device, this is done using the
    > > shared-dma-pool DT attribute, which comes in two variants that would be a
    > > little to much to explain here.
    > >
    > > remoteproc makes use of that because apparently the device can only
    > > communicate using that region. But it then feeds back memory obtained
    > > with dma_alloc_coherent into the virtio code. For that it calls
    > > vmalloc_to_page on the dma_alloc_coherent, which is a huge no-go for the
    > > ĐMA API and only worked accidentally on a few platform, and apparently
    > > arm64 just changed a few internals that made it stop working for remoteproc.
    > >
    > > The right answer is to not use the DMA API to allocate memory from a
    > > device-speficic region, but to tie the driver directly into the DT reserved
    > > memory API in a way that allows it to easilt obtain a struct device for it.
    >
    > Just have a question,
    >
    > Since vmalloc_to_page is ok for cma area, no need to take cma and per device
    > cma into consideration right?
    >
    > we only need to implement a piece code to handle per device specific region
    > using RESERVEDMEM_OF_DECLARE, just like:
    > RESERVEDMEM_OF_DECLARE(rpmsg-dma, "rpmsg-dma-pool",
    > rmem_rpmsg_dma_setup);
    > And implement the device_init call back and build a map between page and phys.
    > Then in rpmsg driver, scatter list could use page structure, no need vmalloc_to_page
    > for per device dma.
    >
    > Is this the right way?

    I CC'ed the rpmsg maintainers, you want to keep them in the loop on
    this.
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