Re: DISABLE_CLUSTERING in scsi drivers

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On Thu, Nov 22, 2018 at 09:02:13AM +1100, Finn Thain wrote:
> > you in the To list maintain or wrote SCSI drivers that set the
> > DISABLE_CLUSTERING flag, which basically disable merges of any
> > bio segments.  We already have the actual max_segment size limit
> > to say which length a segment should have, independent of merged
> > or originally created, so this limit generally should rarely if
> > ever be required, and mostly is an old cut an paste error.
> > 
> 
> Are you referring to
> 	blk_queue_max_segment_size(q, dma_get_max_seg_size(dev));
> in drivers/scsi/scsi_lib.c?
> 
> Is the segment size limitation of the DMA controller the only reason to 
> want DISABLE_CLUSTERING?

DISABLE_CLUSTERING mixes up two not really related things:

 1) limit the size of each segment to a single page size
 2) limit each segment to not actually span a page boundary.

Both could be valid limit for DMA engines, but also might be particularly
relevant for pio, if you e.g. kmap each page of a scatterlist do do
pio you'd want to see both limits.



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