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    SubjectRe: [PATCH v7 3/8] arm64: introduce is_device_dma_coherent
    Hi Stefano,

    On 11/03/2014 01:10 PM, Stefano Stabellini wrote:
    > On Mon, 3 Nov 2014, Will Deacon wrote:
    >> On Mon, Nov 03, 2014 at 10:46:03AM +0000, Stefano Stabellini wrote:
    >>> On Mon, 27 Oct 2014, Stefano Stabellini wrote:
    >>>> Introduce a boolean flag and an accessor function to check whether a
    >>>> device is dma_coherent. Set the flag from set_arch_dma_coherent_ops.
    >>>>
    >>>> Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
    >>>> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
    >>>> CC: will.deacon@arm.com
    >>>
    >>> Will, Catalin,
    >>> are you OK with this patch?
    >>
    >> It would be nicer if the dma_coherent flag didn't have to be duplicated by
    >> each architecture in dev_archdata. Is there any reason not to put it in the
    >> core code?
    >
    > Yes, there is a reason for it: if I added a boolean dma_coherent flag in
    > struct device as Catalin initially suggested, what would be the default
    > for each architecture? Where would I set it for arch that don't use
    > device tree? It is not easy.
    >
    > I thought it would be better to introduce is_device_dma_coherent only on
    > the architectures where it certainly makes sense to have it. In fact I
    > checked and arm and arm64 are the only architectures to define
    > set_arch_dma_coherent_ops at the moment. At that point if
    > is_device_dma_coherent becomes arch-specific, it makes sense to store
    > the flag in dev_archdata instead of struct device.

    The proposition from Will looks reasonable for me too, because
    there is "small" side-effect of adding such kind of properties to
    arch-specific data or even to the core device structure. ;(

    There are some sub-systems in kernel which do not create their devices
    from DT and instead some host device populates its children devices manually.
    Now, I know at least two cases:
    - usb: dwc3 core creates xhci device manually
    - pci: adds its client devices

    In such, case DMA configuration have to be propagated from host to
    child (in our case host device's got DMA configuration from DT), like:
    dma_set_coherent_mask(&xhci->dev, dwc->dev->coherent_dma_mask);

    xhci->dev.parent = dwc->dev;
    xhci->dev.dma_mask = dwc->dev->dma_mask;
    xhci->dev.dma_parms = dwc->dev->dma_parms;

    So, once new DMA property is added it has to be propagated from
    host to child device too.

    Recently, the new property dma_pfn_offset was introduced in struct device
    and such kind of problem was observed on keystone 2:
    - for usb case it was fixed using Platform Bus notifier (xhci - platform device)
    - for pci - the work is in progress, because solution with PCI Bus notifier
    was rejected https://lkml.org/lkml/2014/10/10/308.

    In general, if dma_coherent will belong to struct device then
    such problems will be possible to fix directly in drivers/subsystems:
    xhci->dev.dma_coherent = dwc->dev->dma_coherent;

    But, if it will be arch-specific data then it will be impossible to
    set it without introducing proper and arch-specific setters/getters functions.

    Also, as an idea, we are thinking about introducing something like:
    void dma_apply_parent_cfg(struct device *dev, struct device *parent)
    which will ensure that all DMA configuration properly copied from
    parent to children device. Now it should be (as minimum for ARM):
    dma_mask
    coherent_dma_mask
    dma_parms
    dma_pfn_offset
    dev_archdata->dma_ops
    [dma_coherent]?

    regards,
    -grygorii



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