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    SubjectRe: [PATCH 1/2] vga: implements VGA arbitration on Linux
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    On Thu, 2009-07-16 at 09:48 +0100, Alan Cox wrote:
    >
    > > >> + pdev = vga_default_device();
    > > >
    > > > What if the BIOS provided device was hot unplugged ?
    > >
    > > we just use the pdev as a cookie, if it was hot unplugged we'll
    > > have gotten a callback to remove it from the VGA device list
    > > and the lookup which happens 5 lines later inside the spinlock
    > > will fail.
    >
    > What if I inserted a new device - the allocator might give me a new
    > device with the same pointer if its reusing the same slab entry for
    > that
    > size. Unlikely but given a zillion boxes this starts to occur 8(

    The original proof-of concept draft code I wrote (and this is -very-
    close to it :-) didn't quite handle hotplug of the default device. That
    does need to be sorted.

    But then, I have a hard time finding any useful locking to use vs. PCI
    hotplug, so it's a non trivial matter.

    > Not commented on - but a serious question would be "do we actually
    > care enough or are there really devices with just I/O and just vga
    > memory access used ?"

    I honestly cannot remember why I split the 4 resource types that way
    back then. I have the nagging feeling that I had a good reason to do so
    but it totally eludes me today :-)

    The one main thing I wanted was to keep legacy vs. standard resources so
    that the all the portions of a driver (DDX, DRM, etc...) that use
    standard resources can continue to lock/unlock just these, without
    having to know whether one of the elements disable legacy decoding (or
    is trying to request it again).

    But maybe somebody with a fresh new look on all this will rightfully say
    "fuck it, that's too complicated" and turn the whole thing into a single
    token to pass around.

    > Your cookie validity is suspect I fear.

    Yes, it is sadly.

    > Also holding the device reference means you stop that exact set of
    > resources being reissued too early and
    > you (or clients) scribbling on them through unfortunate timing. So I
    > think you actually do need to grab references properly, Doesn't make
    > the
    > code much more complex that I can see.
    >
    Well, we are in the device addition/removal path, so the list of vga
    devices doesn't need to hold a reference here. However, we probably need
    to make sure we take one when we peek something from that list.

    Cheers,
    Ben.



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