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Date:   Fri, 21 Jun 2019 15:37:11 +0200
From:   Halil Pasic <pasic@linux.ibm.com>
To:     Cornelia Huck <cohuck@redhat.com>
Cc:     Sebastian Ott <sebott@linux.ibm.com>,
        Peter Oberparleiter <oberpar@linux.ibm.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] s390/cio: introduce driver_override on the css bus
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        <20190621115604.0f3e3f69.cohuck@redhat.com>
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On Fri, 21 Jun 2019 11:56:04 +0200
Cornelia Huck <cohuck@redhat.com> wrote:

> On Thu, 13 Jun 2019 13:08:15 +0200
> Cornelia Huck <cohuck@redhat.com> wrote:
> 
> > Sometimes, we want to control which of the matching drivers
> > binds to a subchannel device (e.g. for subchannels we want to
> > handle via vfio-ccw).
> > 
> > For pci devices, a mechanism to do so has been introduced in
> > 782a985d7af2 ("PCI: Introduce new device binding path using
> > pci_dev.driver_override"). It makes sense to introduce the
> > driver_override attribute for subchannel devices as well, so
> > that we can easily extend the 'driverctl' tool (which makes
> > use of the driver_override attribute for pci).
> > 
> > Note that unlike pci we still require a driver override to
> > match the subchannel type; matching more than one subchannel
> > type is probably not useful anyway.
> > 
> > Signed-off-by: Cornelia Huck <cohuck@redhat.com>

Reviewed-by: Halil Pasic <pasic@linux.ibm.com>

I guess the '\n' handling is customary, and is what the same what
the pci counterpart (782a985d7af2) does anyway. It bothers
me a little that you don't necessarily get back from with show
what you stored. E.g. # echo -e "bug\nfree"
> /sys/bus/css/devices/0.0.0001/driver_override # echo $?
0
# cat /sys/bus/css/devices/0.0.0001/driver_override
bug
# echo $?
0
But given the previous art (782a985d7af2) I think it is the best way
to do it.

The rest is very straightforward.

> > ---
> > 
> > Lightly tested; did not yet attempt to adapt driverctl to actually
> > make use of it.
> 
> Friendly ping.
> 
> In the meanwhile, I figured out that you do not need to adapt driverctl
> at all, but just need to pass it '-b css' to work on the css bus; this
> seems to work just fine with this patch applied.
> 

Interesting. I hope to get around and have a closer look at it
eventually.

Regards,
Halil