Re: [PATCH v4 0/2] [SCSI] Asynchronous event notification infrastructure

[Jeff Garzik <jeff@garzik.org>]

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.

	Jeff