linux-kernel - Re: [patch 04/32] genirq/msi: Provide a set of advanced MSI accessors and iterators

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Message-ID: <87a6hndnpz.ffs@tglx>
Date:   Mon, 29 Nov 2021 15:46:00 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Marc Zygnier <maz@...nel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Megha Dey <megha.dey@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-pci@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-s390@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Jon Mason <jdmason@...zu.us>,
        Dave Jiang <dave.jiang@...el.com>,
        Allen Hubbe <allenbh@...il.com>, linux-ntb@...glegroups.com
Subject: Re: [patch 04/32] genirq/msi: Provide a set of advanced MSI
 accessors and iterators

Jason,

On Mon, Nov 29 2021 at 10:01, Jason Gunthorpe wrote:
> On Mon, Nov 29, 2021 at 10:26:11AM +0100, Thomas Gleixner wrote:
>> After looking at all the call sites again, there is no real usage for
>> this local index variable.
>> 
>> If anything needs the index of a descriptor then it's available in the
>> descriptor itself. That won't change because the low level message write
>> code needs the index too and the only accessible storage there is
>> msi_desc.
>
> Oh, that makes it simpler, just use the current desc->index as the
> input to the xa_for_each_start() and then there should be no need of
> hidden state?

That works for alloc, but on free that's going to end up badly.

>> What for? The usage sites should not have to care about the storage
>> details of a facility they are using.
>
> Generally for_each things shouldn't have hidden state that prevents
> them from being nested. It is just an unexpected design pattern..

I'm not seeing any sensible use case for:

     msi_for_each_desc(dev)
        msi_for_each_desc(dev)

If that ever comes forth, I'm happy to debate this further :)

Thanks,

        tglx