On Mon, Nov 26, 2018 at 08:46:39PM +1100, Finn Thain wrote: > On Mon, 26 Nov 2018, Christoph Hellwig wrote: > > > 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. > > > > I looked through all the drivers based on esp_scsi.c and NCR5380.c which > use DISABLE_CLUSTERING. > > There is one driver that uses DMA and sometimes kmap too, which is > am53c974.c. It defaults to ENABLE_CLUSTERING, which would seem to be a bug > if kmap isn't compatible with that (Hannes?). > > For g_NCR5380.c (ISA bus) and dmx3191d.c (PCI bus) these drivers need > DISABLE_CLUSTERING because apparently they don't use kmap or DMA. This > will need to be addressed if you remove DISABLE_CLUSTERING. > > Then we have the ARM drivers, cumana_1.c and oak.c. I believe that ecard > drivers like these are only used with CONFIG_ARCH_RPC, and I think that > would imply !CONFIG_HIGHMEM (Russell?). > > m68k that doesn't seem to have CONFIG_HIGHMEM either, so no need for kmap > or DISABLE_CLUSTERING when doing PIO or PDMA. That includes mac_esp.c and > mac_scsi.c. Michael has already answered for atari_scsi.c. > > That leaves the Sun 3 drivers, sun3_scsi.c and sun3_scsi_vme.c, which may > have DMA limitations I don't know about (Sam?).
I don't believe the Sun3 DMA has limitations which require DISABLE_CLUSTERING as described above. I'm going to go with the "old cut and paste error" theory as being most likely. -- Sam
