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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Fri, 17 Dec 2021 14:32:00 +0100
Message-ID: <CAK8P3a2eZ25PLSqEf_wmGs912WK8xRMuQHik2yAKj-WRQnDuRg@mail.gmail.com>
Subject: Re: [GIT PULL 1/2] asm-generic: rework PCI I/O space access
To:     Niklas Schnelle <schnelle@linux.ibm.com>
Cc:     John Garry <john.garry@huawei.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        linux-pci <linux-pci@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Fri, Dec 17, 2021 at 2:19 PM Niklas Schnelle <schnelle@linux.ibm.com> wrote:
>
> I've had some time to look into this a bit. As a refreshed starting
> point I have rebased Arnd's patch to v5.16-rc5. Since I'm not sure how
> to handle authorship and it's very early I haven't sent it as RFC but
> it's available as a patch from my GitHub here:
> https://gist.github.com/niklas88/a08fe76bdf9f5798500fccea6583e275
>
> I have incorporated the following findings from this thread already:
>
> - Added HAS_IOPORT to arch Kconfigs
> - Added "config LEGACY_PCI" to drivers/pci/Kconfig
> - Fixed CONFIG_HAS_IOPORT typo in asm-generic/io.h
> - Removed LEGACY_PCI dependency of i2c-i801.
>   Which is also used in current gen Intel platforms
>   and depends on x86 anyway.
>
> I have tested this on s390 with HAS_IOPORT=n and allyesconfig as well
> as running it with defconfig. I've also been using it on my Ryzen 3990X
> workstation with LEGACY_PCI=n for a few days. I do get about 60 MiB
> fewer modules compared with a similar config of v5.15.8. Hard to say
> which other systems might miss things of course.
>
> I have not yet worked on the discussed IOPORT_NATIVE flag. Mostly I'm
> wondering two things. For one it feels like that could be a separate
> change on top since HAS_IOPORT + LEGACY_PCI is already quite big.
> Secondly I'm wondering about good ways of identifying such drivers and
> how much this overlaps with the ISA config flag.
>
> I'd of course appreciate feedback. If you agree this is still
> worthwhile to persue I'd think the next step would be trying to
> refactor this into more manageable patches.

Thanks a lot for restarting this work! I think this all looks reasonable
(a lot was my original patch anyway, so of course I think that ;)), but
it would be good to split it up into multiple patches.

The CONFIG_LEGACY_PCI should take care of a lot of it, and I
think that can be a single patch. I'd expand the Kconfig description
to explain that this also covers PCIe devices that use the legacy
I/O space even if they do not have a PCIe-to-PCI bridge in them.

The introduction of CONFIG_HAS_IOPORT, plus selecting it from
the respective architectures makes sense as another patch, but
I would make that separate from the #ifdef and 'depends on'
changes to individual subsystems or drivers, as they are
better reviewed separately.

        Arnd