Re: CD writing in future Linux (stirring up a hornets' nest)

[D. Hazelton]

On Sunday 19 February 2006 04:20, Matthias Andree wrote:
> On Sat, 18 Feb 2006, D. Hazelton wrote:
> > At this point I seem to have stumbled across a document that has what
> > Joerg might be looking for Linux to introduce. It's available at t10.org
> > and is a translation layer specification for OS's to use if they want to
> > access ATA devices like SCSI ones.
>
> Is that something other than CAM? If so, please mention the exact
> document name.


The documents title is: "Information Technology - SCSI / ATA translation (SAT)

The revision I'm looking at is R08 - the most current I could find. I'll have 
to dig, but it doesn't appear to just be a rewrite of CAM from the quick once 
over I gave it.

> > Seems to me this wouldn't be a good or bad thing to add to the kernel.
> > The
>
> Well, such an integration must avoid:
>
> - breaking existing conformant applications, where conformant here would
>   apply to existing interface documentation such as the SCSI General
>   Programming HOWTO from torque.net.
>
>   This means that int fd = open("/dev/foo", O_RDWR, 0);
>                   int e = ioctl(fd, SG_IO, &cmd_block);
>   needs to remain functional.
>
> - duplicating code which would cause immediate bit-rot...

Yes, true. After all Linus has said that there is to be no more breaking of 
userspace interfaces. In this case I think what the layer would do is allow 
the people that want to to use /dev/sg* to access all SCSI and ATA devices on 
a given system.

It's purpose, from the blurb on t10.org, is to provide a SCSI interface to ATA 
disks for people that want to access them that way.

> > problem is that introducing the layer and thereby unifying the SCSI and
> > ATA busses into one namespace is a big task.
>
> ...so it could really only be an interface layer on top of existing
> interfaces to avoid that. (Any other opinions?)
>

Right. Which is one reason why I did say that I thought it might be a good 
idea, but didn't think that anyone would bother. I also don't think it's 
really necessary. SG_IO happens to work great and the amount of work 
involved...

Anyway, to me it seems like it'd just be resurrecting a deprecated module and 
expanding it so it has a wider scope. Only real difference would be that it 
wouldn't take devices from their driver, just provide a second interface to 
it - which also means that all the ATA drivers would need to have hooks that 
it could call into. (Or am I over thinking it here?)

> And then that interface would only be sensible if it actually improves
> the situation, for instance, if applications gain source-level
> compatibility with NetBSD or FreeBSD.

Well, FreeBSD implements CAM, so if someone implemented CAM that'd provide 
source-level compatability. In this case what it would do is hard to say - 
the only thing I can think it _might_ do is quiet a couple of people down a 
bit.

> I think libata's integration of parallel ATA is already going a way that
> leads to unifiying all the layers. For disks, the sd driver is used with
> libata. I'd be surprised if it wouldn't work for CD/DVD drives, too, at
> least in the long run.

Ah, yes. I was under the impression that libata only handled PATA and SATA - 
is it going to be expanded to encompass the entire ATA bus?

> Part of the problem is Jörg's expecting a solution the day before
> yesterday.

Well, from some comments he made in private mails he seems to think he was 
promised (by Linus, no less) that the DMA problems in ide-scsi were going to 
be fixed. Instead the module was deprecated and SG_IO was introduced - this 
seems to be one of the things he's been angry about.

DRH
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