CAM tags / reset problem

[Matthew Jacob]

I'll let Nate or Ken or Justin comment.

On 9/17/05, Ade Lovett <ade at freebsd.org> wrote: 
> 
> The observed behaviour is this: there is a bus reset during device
> probing / negotiation during boot, and after the system is up many of
> the scsi devices are found to be running with dev_openings = 1, which
> is less than the mintags = 2 setting, even though tag queueing is
> supposedly enabled. Performance is impaired as a result.
> 
> The cause appears to be as follows: in cam_xpt.c, the routine
> xpt_dev_ccbq_resize saves, in dev->tag_saved_openings, the requested
> queue size _if and only if_ tag queueing is either enabled (SID_CmdQue
> is set) or is _scheduled_ to be enabled (CAM_DEV_TAG_AFTER_COUNT is
> set). However, the following sequence can occur:
> 
> 1) Device transfer settings are negotiated. The device can support
> tags, so CAM_DEV_TAG_AFTER_COUNT is set (tag queueing is _not_
> immediately enabled).
> 
> 2) Before enough commands have been sent to cause tag queueing to be
> started, the bus is reset and all transfer settings are reset. This
> results in xpt_set_transfer_settings being called, and in turn this
> clears SID_CmdQue and calls xpt_dev_ccbq_resize (to resize the queue
> down to its non-tagged size, typically 1), and only then clears
> CAM_DEV_TAG_AFTER_COUNT (which was of course set) and its associated
> count. This causes xpt_dev_ccbq_resize to save the requested queue
> size even though it does not relate to the desired size with tag
> queueing enabled.
> 
> 3) If tag-queueing is enabled again, e.g. after another negotiation,
> then the saved value (1) of tag_saved_openings is used rather than
> the correct value.
> 
> The fix seems to be to clear CAM_DEV_TAG_AFTER_COUNT _before_ resizing
> the queue, as per the following patch:
> 
> http://people.FreeBSD.org/~ade/sys-dev-cam-xpt.c
> 
> I'd like permission to commit this to HEAD, followed by a relatively
> quick MFC to RELENG_6 (I know I've missed BETA5, but would like to
> get it into -RELEASE)
> 
> -aDe
> 
> _______________________________________________
> freebsd-scsi at freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-scsi
> To unsubscribe, send any mail to "freebsd-scsi-unsubscribe at freebsd.org"
>