On Tue, Nov 15 2005, Jeff Garzik wrote:
For departure of libata from SCSI, I was thinking more of another more generic block device framework in which libata can live in. And I thought that it was reasonable to assume that the framework would supply a EH mechanism which supports queue stalling/draining and separate thread. So, my EH patches tried to make the same environment for libata
A big reason why libata uses the SCSI layer is infrastructure like this. It would certainly be nice to see timeouts and EH at the block layer. The block layer itself already supports queue stalling/draining.
I have a pretty simple plan for this:
- Add a timer to struct request. It already has a timeout field for
SG_IO originated requests, we could easily utilize this in general.
I'm not sure how the querying of timeout would happen so far, it would
probably require a q->set_rq_timeout() hook to ask the low level
driver to set/return rq->timeout for a given request.
- Add a timeout hook to struct request_queue that would get invoked from
the timeout handler. Something along the lines of:
- Timeout on a request happens. Freeze the queue and use
kblockd to take the actual timeout into process context, where
we call the queue ->rq_timeout() hook. Unfreeze/reschedule
queue operations based on what the ->rq_timeout() hook tells
us.
That is generic enough to be able to arm the timeout automatically from
->elevator_activate_req_fn() and dearm it when it completes or gets
deactivated. It should also be possible to implement the SCSI error
handling on top of that.