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

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Message-ID: <877dcsf5l5.ffs@tglx>
Date:   Sun, 28 Nov 2021 20:22:30 +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

On Sat, Nov 27 2021 at 21:00, Jason Gunthorpe wrote:
> On Sat, Nov 27, 2021 at 02:22:33AM +0100, Thomas Gleixner wrote:
>> + * Notes:
>> + *  - The loop must be protected with a msi_lock_descs()/msi_unlock_descs()
>> + *    pair.
>> + *  - It is safe to remove a retrieved MSI descriptor in the loop.
>> + */
>> +#define msi_for_each_desc_from(desc, dev, filter, base_index)			\
>> +	for ((desc) = __msi_first_desc((dev), (filter), (base_index)); (desc);	\
>> +	     (desc) = msi_next_desc((dev)))
>
> Given this ends up as an xarray it feels really weird that there is a
> hidden shared __next/__iter_idx instead of having the caller provide
> the index storage as is normal for xa operations.
>
> I understand why that isn't desirable at this patch where the storage
> would have to be a list_head pointer, but still, seems like an odd
> place to end up at the end of the series.
>
> eg add index here unused and then the last patch uses it instead of
> __iter_idx.

TBH, didn't think about doing just that. OTH, given the experience of
looking at the creative mess people create, this was probably also a
vain attempt to make it harder in the future.

> Also, I don't understand why filter was stored in the dev and not
> passed into msi_next_desc() in the macro here?

No real reason. I probably just stored it along with the rest. Lemme try
that index approach.

Thanks,

        tglx