Date:   Wed, 1 Dec 2021 14:47:26 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Logan Gunthorpe <logang@deltatee.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Marc Zygnier <maz@kernel.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        Kevin Tian <kevin.tian@intel.com>,
        Megha Dey <megha.dey@intel.com>,
        Ashok Raj <ashok.raj@intel.com>, linux-pci@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jon Mason <jdmason@kudzu.us>,
        Dave Jiang <dave.jiang@intel.com>,
        Allen Hubbe <allenbh@gmail.com>, linux-ntb@googlegroups.com,
        linux-s390@vger.kernel.org, Heiko Carstens <hca@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>, x86@kernel.org
Subject: Re: [patch 21/32] NTB/msi: Convert to msi_on_each_desc()
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On Wed, Dec 01, 2021 at 07:37:32PM +0100, Thomas Gleixner wrote:
> On Wed, Dec 01 2021 at 11:11, Jason Gunthorpe wrote:
> > On Wed, Dec 01, 2021 at 03:52:02PM +0100, Thomas Gleixner wrote:
> >> So we really can go and create a MSI irqdomain and stick the pointer
> >> into stdev->dev.irqdomain. The parent domain of this irqdomain is
> >> 
> >>      stdev->pdev.dev.irqdomain->parent
> >
> > It can work (pending some solution to the iommu stuff), but IMHO it is
> > strange/hacky to put HW objects like irqdomain on what is a character
> > struct device with a set major/minor in dev->devt and associated
> > struct cdev.
> >
> > Conceptually it makes no sense to me, cdevs are software constructs,
> > they should never go into HW areas..
> 
> I picked that because it _is_ already used to establish the connection
> to the switchtec_class NTB driver which is beyond the usual cdev muck.

IMHO that is also a misuse. These days two drivers should be hooked
together using an aux device, not a cdev and the obscure
class_interface stuff. Aux device supports auto probing and module
auto loading for instance.

An interrupt on an aux device is at least somewhat conceptually
parallel to an interrupt on a mdev as both are usually representing
some slice of a device.

Thanks,
Jason