CAM tags / reset problem

[Ade Lovett]

Any further thoughts on this, chaps?

Unless anyone objects strenuously, I'd like to go ahead and commit  
this to HEAD, give it a month or so to settle in, and then MFC to  
both RELENG_6 and RELENG_5.

-aDe

On Sep 18, 2005, at 22:39 , Matthew Jacob wrote:

> 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
>>
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