linux-kernel - RE: [patch V2 02/33] genirq/msi: Provide struct msi_parent

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Message-ID: <BN9PR11MB52763BEF1A4C88368B8A9E448C0F9@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Thu, 24 Nov 2022 00:53:29 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
CC:     "x86@...nel.org" <x86@...nel.org>, Joerg Roedel <joro@...tes.org>,
        "Will Deacon" <will@...nel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Lorenzo Pieralisi" <lorenzo.pieralisi@....com>,
        Marc Zyngier <maz@...nel.org>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        Jason Gunthorpe <jgg@...lanox.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        "Raj, Ashok" <ashok.raj@...el.com>, Jon Mason <jdmason@...zu.us>,
        Allen Hubbe <allenbh@...il.com>
Subject: RE: [patch V2 02/33] genirq/msi: Provide struct msi_parent_ops

> From: Thomas Gleixner <tglx@...utronix.de>
> Sent: Wednesday, November 23, 2022 7:29 PM
> 
> On Wed, Nov 23 2022 at 07:57, Kevin Tian wrote:
> >> From: Thomas Gleixner <tglx@...utronix.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?
> 

yes. with the hierarchy being arch specific those dependencies
can be easily figured out.