Re: [RFC PATCH 04/12] s390/cio: introduce cio DMA pool

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 10 Apr 2019 17:31:48 +0200
Halil Pasic <pasic@linux.ibm.com> wrote:

> On Tue, 9 Apr 2019 19:14:53 +0200
> Cornelia Huck <cohuck@redhat.com> wrote:
> 
> [..]
> 
> > > > At this point, you're always going via the css0 device. I'm
> > > > wondering whether you should pass in the cssid here and use
> > > > css_by_id(cssid) to make this future proof. But then, I'm not
> > > > quite clear from which context this will be called.      
> > > 
> > > As said before I don't see MCSS-E coming. Regarding the client code,
> > > please check out the rest of the series. Especially patch 6. 
> > > 
> > > From my perspective it would be at this stage saner to make it more
> > > obvious that only one css is supported (at the moment), than to
> > > implement MCSS-E, or to make this (kind of) MCSS-E ready.    
> > 
> > I disagree. I think there's value in keeping the interfaces clean
> > (within reasonable bounds, of course.) Even if there is no
> > implementation of MCSS-E other than QEMU... it is probably a good idea
> > to spend some brain cycles to make this conceptually clean.  
> 
> AFAIU Linux currently does not support MCSS-E. I don't have the
> bandwidth to implement MCSS-E support in the kernel.
> 
> I fully agree for external interfaces should be MCSS-E conform, so
> should we ever decide to implement we don't have to overcome self-made
> obstacles.
> 
> Kernel internal stuff however, IMHO, should avoid carrying a ballast of
> an 20%-30% implemented MCSS-E support. I see no benefit.
> 
> But I don't insist. If you have a good idea how to make this more MCSS-E
> conform, you are welcome. In think something like this is best done on
> top of a series that provides a basic working solution. Especially if the
> conceptually clean thing is conceptually or code-wise more complex than
> the basic solution. If you have something in mind that is simpler and
> more lightweight than what I did here, I would be more than happy to make
> that happen.

I haven't asked for adding complete support for MCSS-E, just to keep it
in the back of our minds...

Back to the issue at hand. You're currently hard-wiring this to the
css0 device. My question is: Does it make sense to have a per-css area?
From my limited perspective, I think it does (more css images would
mean more devices, and just adding smaller areas per-css is probably
easier than adding one big area scaling with the number of css images).
In that case, what about tacking the area to the actual css object,
instead of having the global variable? Should be an easy change, and
feels cleaner.