Re: PATCH [5/15] qla2xxx: SG tablesize update

[Anton Blanchard <anton@samba.org>]

 
> Actually, to be fair, I think he means the qlogic request queue, not the
> block one.
> 
> The Qla chips are rather weird in that they have a single issue queue
> (whose size you can vary) but whose entry formats are fixed.  If I
> remember correctly, an initial command can have 4 SG elements, but a
> follow on entry can have 7 (not sure of the figures).  But anyway, large
> SG commands end up having to find multiple entries in this queue (and
> being a single issue queue for the entire card, it has to be mutexed
> while you search).  The more resources you need, the more difficult the
> search and the more contention you generate on the resource mutex.

With 2.6.3 on a ppc64 box connected to a shark with 8 LUNs I could
easily consume all the request slots on the card. At this point we spent
> 50% of the cpu bouncing into qla2x00_start_scsi, doing a pci_map_sg
realising it wont fit and doing a pci_unmap_sg.

As Andrew points out, the driver shouldnt be doing this constant
map/unmap stuff, but is there some way for a scsi device driver to tell
the layers above it to temporarily leave it alone?

Anton