Jeff Garzik wrote:
Weak ACK for the patchset:
SET FEATURES - XFER MODE also needs to use the defer capability.
We need to reconfigure the controller and revalidate devices after XFER
MODE, so simple command exclusion isn't enough for XFER MODE. I think
we need EH level synchronization. e.g. Something like the following.
* Controller has two ports portA and portB. It needs host-wide
exclusion for XFER_MODE and the associated controller reprogramming.
1. User (or whatever) issues XFER MODE for portA.
2. portA EH is invoked for XFER MODE. (local exclusion automatically
achieved)
3. portA EH asks portB EH to participate in host-wide exclusion.
4. portB EH is invoked and waits for port A EH to complete.
5. portA EH now has host-wide exclusion. Do XFER MODE, reprogram
controller and revalidate.
6. portA EH tells portB EH that it's free to go and portA resumes.
7. portB resumes.
And we also have such a need for simplex devices. It would be useful
to have all of this stuff collected into an "ATA command scheduling /
sequencing" area, rather than just a new hook confined to a single
area. I think the problem space is more complex than that.
For simplex controllers, I dunno. The current code depends on SCSI
midlayer for command queue management and, as each ATA port is
represented as separate SCSI host, it's difficult to implement
cross-host exclusion properly. Also, are simplex controllers big deal?
Aren't they mostly historical?
So, although per-host scheduling by asking SCSI midlayer to retry on
certain conditions is a bit crude, I think it covers most common use
cases (NCQ, PMP and whatever port level exclusion).
As you pointed out previously, it would be better if we can do it via
return value of ->qc_prep or ->qc_issue, but we can't because a prepped
qc cannot be given back to SCSI queue for retrial, which makes the
separation between ->qc_prep and ->qc_issue meaningless at the moment.
So, we end up with, ->qc_defer (before qc prep) and ->qc_prep/issue
(after qc prep).
Thanks.
--
tejun
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