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    SubjectRe: [PATCH v4 0/2] [SCSI] Asynchronous event notification infrastructure
    On Mon, 2007-10-29 at 11:58 -0400, Jeff Garzik wrote:
    > James Bottomley wrote:
    > > On Mon, 2007-10-29 at 10:42 -0400, Jeff Garzik wrote:
    > >> This is the next revision of the SCSI event notification infrastructure
    > >> patchset, enabling SATA Asynchronous Notification ("AN") for CD/DVD
    > >> devices that support it.
    > >>
    > >> For devices that support SATA AN (only very recent ones do), this means
    > >> that HAL and other userspace utilities no longer need to repeatedly poll
    > >> the CD/DVD device to determine if the user has changed the media.
    > >>
    > >> This revision takes into account James' comments from earlier today,
    > >> modulo the following notes:
    > >>
    > >> * I think the various event attributes should always be present,
    > >> for all devices at all times. If various events are not supported,
    > >> the attribute will of course return zero (false, not supported).
    > >
    > > Actually, I don't think so. We have precedent for this in the transport
    > > classes: if a device doesn't support a feature, we don't export the flag
    > > for that feature through sysfs. This allows not only feature control,
    > > but an immediate view of the device capabilities simply by viewing the
    > > sysfs directory.
    > Think about about the values being exported by these sysfs attributes:
    > they indicate whether or not that feature is supported.

    Ah, OK; I haven't communicated what we need very clearly. We need a way
    to see if the event is supported by the device, as well as a way to turn
    it off. For some of the events (possibly not the SATA AN one, since I
    know all SATA devices will be well behaved) there's going to be a need
    to deal with berserk or broken devices that become trigger happy, so
    turning off the event will be a useful (and possibly essential) way of

    > Thus, using the presence/absence of an attribute to communicate the same
    > thing would be redundant.
    > This suggestion adds a whole lot of complexity -- mirroring every change
    > to sdev->supported_events by dynamically adding or removing attributes.
    > The current nice, simple, elegant bitops-based interface is suddenly a
    > lot more cumbersome if forced to deal with attribute creation and disposal.
    > Finally, this additional complexity of dynamic attribute management also
    > eliminates some key information: userland can test the existence of the
    > attribute to determine if that support is present in the kernel.


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