linux-kernel - Re: [patch RFC 30/38] PCI/MSI: Allow to disable arch fallbacks

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Message-ID: <87a6yixvnl.fsf@nanos.tec.linutronix.de>
Date:   Tue, 25 Aug 2020 23:28:30 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        Joerg Roedel <joro@...tes.org>,
        iommu@...ts.linux-foundation.org, linux-hyperv@...r.kernel.org,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Jon Derrick <jonathan.derrick@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Wei Liu <wei.liu@...nel.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Steve Wahl <steve.wahl@....com>,
        Dimitri Sivanich <sivanich@....com>,
        Russ Anderson <rja@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        xen-devel@...ts.xenproject.org, Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Megha Dey <megha.dey@...el.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jacob Pan <jacob.jun.pan@...el.com>,
        Baolu Lu <baolu.lu@...el.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [patch RFC 30/38] PCI/MSI: Allow to disable arch fallbacks

On Tue, Aug 25 2020 at 15:07, Bjorn Helgaas wrote:
>> + * The arch hooks to setup up msi irqs. Default functions are implemented
>> + * as weak symbols so that they /can/ be overriden by architecture specific
>> + * code if needed.
>> + *
>> + * They can be replaced by stubs with warnings via
>> + * CONFIG_PCI_MSI_DISABLE_ARCH_FALLBACKS when the architecture fully
>> + * utilizes direct irqdomain based setup.
>
> Do you expect *all* arches to eventually use direct irqdomain setup?

Ideally that happens some day. We have five left when x86 is converted:

IA64, MIPS, POWERPC, S390, SPARC

IA64 is unlikely to be fixed, but might be solved naturally by removal.

For the others I don't know, but it's not on the horizon anytime soon I
fear.

> And in that case, to remove the config option?

Yes, and all the code which depends on it.

> If not, it seems like it'd be nicer to have the burden on the arches
> that need/want to use arch-specific code instead of on the arches that
> do things generically.

Right, but they still share the common code there and some of them
provide only parts of the weak callbacks. I'm not sure whether it's a
good idea to copy all of this into each affected architecture.

Or did you just mean that those architectures should select
CONFIG_I_WANT_THE CRUFT instead of opting out on the fully irq domain
based ones?

Thanks,

        tglx