linux-kernel - Re: [PATCH] iommu/io-pgtable: Make IOMMU_IO_PGTABLE

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Message-ID: <CAMuHMdV-MbjyoyVM4_aAYs18L4i+2XbeQvRoMChzvsapX+1TKQ@mail.gmail.com>
Date:   Tue, 27 Sep 2022 17:55:46 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Sven Peter <sven@...npeter.dev>, Janne Grunau <j@...nau.net>,
        Hector Martin <marcan@...can.st>, iommu@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/io-pgtable: Make IOMMU_IO_PGTABLE_DART invisible

Hi Robin,

On Tue, Sep 27, 2022 at 5:50 PM Robin Murphy <robin.murphy@....com> wrote:
> On 2022-09-27 16:29, Geert Uytterhoeven wrote:
> > On Tue, Sep 27, 2022 at 5:09 PM Robin Murphy <robin.murphy@....com> wrote:
> >> On 2022-09-27 15:48, Geert Uytterhoeven wrote:
> >>> On Tue, Sep 27, 2022 at 4:15 PM Robin Murphy <robin.murphy@....com> wrote:
> >>>> On 2022-09-27 14:36, Geert Uytterhoeven wrote:
> >>>>> There is no point in asking the user about both "Apple DART Formats" and
> >>>>> "Apple DART IOMMU Support", as the former is useless without the latter,
> >>>>> and the latter auto-selects the former.
> >>>>>
> >>>>> Fixes: 745ef1092bcfcf3b ("iommu/io-pgtable: Move Apple DART support to its own file")
> >>>>> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
> >>>>> ---
> >>>>> Should IOMMU_IO_PGTABLE_LPAE and IOMMU_IO_PGTABLE_ARMV7S be made
> >>>>> invisible, too?
> >>>>> Are there users that do not select them?
> >>>>
> >>>> The aim was for formats to be independently selectable for COMPILE_TEST
> >>>> coverage. The Arm formats are manually selectable for the sake of their
> >>>> runtime self-tests, which are self-contained, but since DART format
> >>>> doesn't do anything by itself I'd agree there's no need to prompt when
> >>>> !COMPILE_TEST here.
> >>>
> >>> IOMMU_IO_PGTABLE_LPAE and IOMMU_IO_PGTABLE_ARMV7S are
> >>> selected by other symbols that can be enabled when compile-testing, so
> >>> the tests can still be enabled in those cases, too
> >>
> >> Sure, but when you want to compile-test a thing, what would you rather
> >> do: enable the thing, or go hunting to find some other thing that
> >> happens to select the thing you actually want, then potentially have to
> >> figure out *that* thing's dependencies, and so on?
> >
> > Agreed.
> >
> >> Coverage isn't solely about whether it's technically possible to ever
> >> reach somewhere at all, it's just as much about how easily and/or often
> >> you can get there in practice. I don't see who benefits from making
> >> COMPILE_TEST harder to use :/
> >
> > So perhaps the visibility of IOMMU_IO_PGTABLE_LPAE and
> > IOMMU_IO_PGTABLE_ARMV7S should depend on COMPILE_TEST?
> > Normal users would still get it through select when needed.
>
> As I say those still offer functionality beyond compile-testing, but now
> you've got me suspecting that it's already suboptimal that one has to
> enable the format to make the self-test option appear... Perhaps what we

IMHO that's the only sensible thing to do: you want to have the option
to enable (preferably modular) tests for the functionality you have
enabled in your product, so you can run the tests when needed.

> want is a separate master option to enable io-pgtable self-tests in
> general, then rejig the rest around that.

IMHO you do not want a master test option that suddenly enables
lots of functionality you do not need in your product.

> Of course the self-tests would be even more useful if the harness was at
> the level of the core io-pgtable API so it could cover new formats
> automatically as long as they provide the configuration parameters, but
> that's a separate matter for someone with sufficient free time and
> enthusiasm :)

OK, I will shut up ;-)

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds