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    SubjectRe: [PATCH 3/3] DT: proc: Add runtime overlay interface in /proc
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    Hi Ionut,

    On Nov 6, 2013, at 11:51 AM, Ionut Nicu wrote:

    > Hi,
    >
    > On 05.11.2013 19:41, ext Pantelis Antoniou wrote:
    >> Add a runtime interface to /proc to enable generic device tree overlay
    >> usage.
    >>
    >> Two new /proc files are added:
    >>
    >> /proc/device-tree-overlay & /proc/device-tree-overlay-status
    >>
    >> /proc/device-tree-overlay accepts a stream of a device tree objects and
    >> applies it to the running kernel's device tree.
    >>
    >> $ cat ~/BB-UART2-00A0.dtbo >device-tree-overlay
    >> overlay_proc_release: Applied #2 overlay segments @0
    >>
    >> /proc/device-tree-overlay-status displays the the overlays added using
    >> the /proc interface
    >>
    >> $ cat device-tree-overlay-status
    >> 0: 861 bytes BB-UART2:00A0
    >>
    >> The format of the status line is
    >> <ID>: <SIZE> bytes <part-number>:<version>
    >>
    >> <ID> is the id of the overlay
    >> <SIZE> is the size of the overlay in bytes
    >> <part-number>, <version> are (optional) root level properties of the DTBO
    >>
    >> You can remove an overlay by echoing the <ID> number of the overlay
    >> precedded with a '-'
    >>
    >> So
    >> $ echo "-0" >device-tree-overlay-status
    >>
    >
    > Wouldn't it be easier if echo "-BB-UART2-00A0" > device-tree-overlay-status was
    > supported also? That way one doesn't need to know the order in which the
    > overlays were applied or parse the status file to get the <ID>.
    >

    Unfortunately no since this is a raw bytestream interface; there is no file
    information. The patchset does display any root level part-number & version properties
    but that's completely options.

    We could standardize in a named root property to use on each overlay, and the part-number &
    version are as good as any.



    >> Removes the overlay.
    >>
    >> Note that this seldom works on most platforms since platform_device
    >> removal is something that almost never works without extra patches.
    >>
    >> Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>
    >
    > It would be very helpful to me if I would have a notification mechanism
    > for overlay add and remove operations based on blocking_notifier_call_chain(),.
    > This way other drivers can be notified when the dt changes.
    >

    Yes. Some people expressed interest in something similar.

    > But I guess that could be added in the future with another patch.
    >

    Yep.

    > Thanks,
    > Ionut

    Let's get the basic support in now, and we can fix all of that on later patches.

    Regards

    -- Pantelis



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