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.
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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