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    SubjectRe: [PATCH] Disable Bus Master on PCI device shutdown
    > This protects against the case where a piece of hardware is continuing 
    > to DMA even after the driver shutdown method has been called? I'm not

    It doesn't. We also have hardware which craps itself if you clear the bus
    mastering bit and we have platforms where the BIOS gets most upset if you
    do that on suspend paths. There are also lots of devices that simply
    ignore the bus mastering bit !

    > convinced this is safe. Some Broadcom parts will crash if busmastering
    > is disabled while they're still performing DMA, and they'll then hang
    > the bus if reenabled.

    This is very common, disabling the bus master bit isn't exactly a
    tested or well defined pathway. A lot of device FIFOs will also happily
    belch out their queue when you flip the bit. So it might look like
    progress but it probably isn't.

    Unfortunately if you've got a device peeing into memory you need to fix
    the driver, or sometimes the firmware. You can probably use the IOMMU for
    some protection on newer systems.

    And then you get things like the CS5520 where disabling bus mastering on
    the IDE controller crashes the machine, and some UMA video devices that
    honour the bit for video fetch.

    From the IDE experience you really need to be careful here. Changing the
    default is asking for nasty and weird regressions.

    This isn't a fix, its a band aid to cover over broken driver shutdown
    methods. Those driver shutdown methods need fixing.


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      Last update: 2012-06-06 23:21    [W:0.024 / U:1.296 seconds]
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