linux-kernel - Re: [patch 19/33] genirq/msi: Provide msi_desc::msi

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Message-ID: <Y3uzVJ/7ecL8DBK7@nvidia.com>
Date:   Mon, 21 Nov 2022 13:20:20 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.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>,
        Dave Jiang <dave.jiang@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Ashok Raj <ashok.raj@...el.com>, Jon Mason <jdmason@...zu.us>,
        Allen Hubbe <allenbh@...il.com>,
        "Ahmed S. Darwish" <darwi@...utronix.de>,
        Reinette Chatre <reinette.chatre@...el.com>
Subject: Re: [patch 19/33] genirq/msi: Provide msi_desc::msi_data

On Fri, Nov 18, 2022 at 11:08:55PM +0100, Thomas Gleixner wrote:

> I looked into this and it gets ugly very fast.
> 
> The above has two parts:
> 
>     iobase    is domain specific and setup by the domain code
> 
>     cookie    is per interrupt allocation. That's where the instance
>               queue or whatever connects to the domain.
> 
> I can abuse the fields for PCI/MSI of course, but see below.

I don't know that we need to store the second one forever in the desc.
I was thinking this information is ephemeral, just used during alloc,
and if the msi domain driver wishes some of it to be stored then it
should do so.

> Sure I could make both cookies plain u64, but I hate these forced type
> casts and the above is simple to handle and understand.

I guess, they aren't what I think of as cookies, so I wouldn't make
them u64 in the first place.

The argument to msi_domain_alloc_irq_at() ideally wants to be a
per-domain-type struct so we can folow it around more cleanly. This is
C so we have to type erase it as a void * through the core code, but
OK.

The second one is typically called "driver private data" in device
driver subsystems that can't use container_of for some reason - just a
chunk of data the driver can associate with a core owned struct.

The usual pattern for driver private data is for the core to provide
some kind of accessor void *get_priv() (think dev_get_drvdata()) or
whatever.

But I do understand your point about keeping the drivers away from
things. Maybe some other pattern is better in this case.

Jason