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From:   Thomas Gleixner <tglx@linutronix.de>
To:     "Tian, Kevin" <kevin.tian@intel.com>,
        LKML <linux-kernel@vger.kernel.org>
Cc:     "x86@kernel.org" <x86@kernel.org>, Joerg Roedel <joro@8bytes.org>,
        Will Deacon <will@kernel.org>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Marc Zyngier <maz@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jason Gunthorpe <jgg@mellanox.com>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        "Williams, Dan J" <dan.j.williams@intel.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        "Raj, Ashok" <ashok.raj@intel.com>, Jon Mason <jdmason@kudzu.us>,
        Allen Hubbe <allenbh@gmail.com>
Subject: RE: [patch V2 02/33] genirq/msi: Provide struct msi_parent_ops
In-Reply-To: <BN9PR11MB52768F0E5B9A4D8E2CF9C5578C0C9@BN9PR11MB5276.namprd11.prod.outlook.com>
References: <20221121083657.157152924@linutronix.de>
 <20221121091326.604272160@linutronix.de>
 <BN9PR11MB52768F0E5B9A4D8E2CF9C5578C0C9@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Wed, 23 Nov 2022 12:28:59 +0100
Message-ID: <878rk1gb9g.ffs@tglx>
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On Wed, Nov 23 2022 at 07:57, Kevin Tian wrote:
>> From: Thomas Gleixner <tglx@linutronix.de>
>> + * One solution is to let the root domain handle the initialization that's
>> + * why there is the @domain and the @msi_parent_domain pointer.
>  
> This is the part which I don't quite understand (sorry with limited knowledge
> in this area).
>
> In concept a hierarchical model has restrictions added up when moving
> down to lower layers i.e. presumably the root domain decides the minimal
> supported capabilities. In this case there is no need of a real parent pointer
> as long as every domain in the stack incrementally adds its restrictions to
> info->flags.
>
> I can see why this is required for x86 given that MULTI_MSI is supported
> only with IR. and we cannot make vector domain inclusively claiming
> MULTI_MSI since it's completely broken when the vector domain becomes
> the parent itself, in absence of IR.
>
> Just be curious whether this intermediate-parent-deciding-restrictions
> is generic instead of x86 specific, e.g. is it possible to have a 4-layers
> hierarchy where the root parent wants to check both two intermediate
> parents?

Sure. Nothing prevents you from doing so:

dom4:
    .init... = dom4_init

dom4_init()
    do_stuff()
    invoke parent init

dom3:
    .init... = parent_init

dom2:
    .init... = dom2_init

dom2_init()
    do_stuff()
    invoke parent init

....

See?

Thanks,

        tglx