Re: [PATCH RFC v3 16/22] ata: libata-scsi: Allocate sdev early in port probe

[John Garry]

> > > > >    @@ -4289,26 +4294,16 @@ void ata_scsi_scan_host(struct ata_port
> > > > > *ap, int sync)
> > > > >     repeat:

I've been trying to follow this thread, below, but got a bit lost ....

> > > > >        ata_for_each_link(link, ap, EDGE) {
> > > > >            ata_for_each_dev(dev, link, ENABLED) {
> > > > > -            struct scsi_device *sdev;
> > > > > +            struct Scsi_Host *shost = ap->scsi_host;
> > > > >                int channel = 0, id = 0;
> > > > >    -            if (dev->sdev)
> > > > > -                continue;
> > > > > -
> > > > >                if (ata_is_host_link(link))
> > > > >                    id = dev->devno;
> > > > >                else
> > > > >                    channel = link->pmp;
> > > > >    -            sdev = __scsi_add_device(ap->scsi_host, channel, 
> > > > > id, 0,
> > > > > -                         NULL);
> > > > > -            if (!IS_ERR(sdev)) {
> > > > > -                dev->sdev = sdev;
> > > > > -                ata_scsi_assign_ofnode(dev, ap);
> > > >
> > > > Is there something equivalent to what this function does inside
> > > > scsi_scan_target() ? I had a quick look but did not see anything...

So are we discussing below whether we can have fixed channel, id, lun 
per ATA sdev per shost? If so, I don't think it would work as libsas 
uses dynamic target ids per host.

> > > Typically, the SCSI layer has two ways of scanning.
> > > One it the old-style serial scanning (originating in the old SCSI
> > > parallel model):
> > > The scanning code will blindly scan _all_ devices up to max_luns, and
> > > attach every device for which the scanning code returns 'OK'.
> > > The other one is to issue REPORT_LUNS and scan all LUNs returned there.
> > >
> > > Mapped to libata we would need to figure out a stable SCSI enumeration,
> > > given that we have PMP and slave devices to content with.
> > > To my knowledge we have ATA ports, each can have either a 'master' and
> > > 'slave' device, _or_ it be a PMP port when it can support up to 16
> > > devices, right?
> >
> > yes
> >
> > > Point being, master/slave and PMP are exclusive, right?
> >
> > Never heard of pmp with ide cable :)
> See?
>
> > > So we can make the master as LUN 0, and the slave as LUN 1.
> >
> > Yes, but isn't that a little wrong ? That would assume that the ata port
> > is the device and the ata devices the luns of that device. But beside
> > the "link busy" stuff that needs to be dealt with, master and slave are
> > independent devices, unlike LUNs. No ?
> > Well; technically, yes.
>
> But we already enumerate the ata ports (which is typically done by the 
> hardware/PCI layer etc), and if we were try to model slave devices as 
> independent ports we would either have to insert a numbering (awkward) 
> or add a number at the en (even more awkward).
>
> And the one key takeaway from the 'multiple actuators' discussion is 
> that LUNs _are_ independent (cf all the hoops they had to jump through 
> to define a command spanning several LUNs ...)(which, incidentally, we 
> could leverage here, too ...), and the target port really only serves as 
> an enumeration thingie to address the LUNs.
>
> So we _could_ map the master device on LUN 0 and the slave device on LUN 
> 1 with no loss of functionality, _but_ enable a consistent SCSI enumeration

Thanks,
John