Jeff Garzik wrote:
Douglas Gilbert wrote:
However, the block layer is used in the context of a
block device (and in some cases a char device).
If SAS domain discovery is done from the user space, and
the root file system is the far side of a SAS expander,
there are no suitable devices, just the SAS initiator
(HBA) which currently we cannot address via the block layer.
Invalid example. All of the methods listed -- request_queue, netlink,
chrdev, sysfs, ioctl -- will work just fine when the root filesystem is
on the far side of a SAS expander. These are just methods of
communication, nothing more.
In your example -- userspace discovery required before root filesystem
can be found -- a program running from initrd/initramfs would create an
SMP device node, open it, and then proceed with the discovery and
configuration process, which in turn creates the device nodes necessary
to mount the root filesystem.
A request_queue is just a queue. You are in complete control of who are
the producer(s) of requests, and who are consumer(s).
Since people are having such a tough time grasping the use of
request_queue without an associated block device, here is a concrete
example: drivers/block/sx8.c.
sx8 creates a queue (grep for 'oob_q') specifically for handling
discovery and configuration requests. The only requests sent to this
queue are I2O-ish management commands, never reads or writes. Since
they are management commands, these requests are NEVER associated with a
block device. Further, when sx8 discovery begins, sx8 block devices
(and associated request_queues) simply don't exist.
Although sx8 management is entirely in-kernel, one could easily imagine
how a userland interface (chrdev?) submits userspace commands into this
queue. Further, one can see how a host adapter could register one or
more queues specifically for the transit of SMP commands.
NOTE: THIS IS NOT AN ENDORSEMENT OF REQUEST QUEUES FOR SMP. I merely
wish to clear up misunderstandings about the block layer found in this
thread.
It remains an open question whether the _complexity_ of this approach is
more than is warranted for SMP. But we've departed from that question,
in this sub-thread :)
I merely illustrate that the block layer is being used _today_ for
management commands.
Jeff
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