linux-kernel - Re: [PATCH] iommu/iova: Fix module config properly

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Message-ID: <YyGjPoM6ooNeJKhT@orome>
Date:   Wed, 14 Sep 2022 11:47:42 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     John Garry <john.garry@...wei.com>, joro@...tes.org,
        will@...nel.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/iova: Fix module config properly

On Tue, Sep 13, 2022 at 03:15:18PM +0100, Robin Murphy wrote:
> On 2022-09-13 14:01, John Garry wrote:
> > On 13/09/2022 12:47, Robin Murphy wrote:
> > > IOMMU_IOVA is intended to be an optional library for users to select as
> > > and when they desire. Since it can be a module now, this means that
> > > built-in code which has chosen not to select it should not fail to link
> > > if it happens to have selected as a module by someone else. Replace
> > > IS_ENABLED() with IS_REACHABLE() to do the right thing.
> > 
> > Hi Robin,
> > 
> > Recently you mentioned "I wonder if we couldn't replace the IS_ENABLED()
> > with IS_REACHABLE() and restore some of the previously-conditional
> > selects", and pointed me to 84db889e6d82 as an example of when a
> > conditional select was made unconditional.
> > 
> > So will you also restore some previously-conditional selects next?
> 
> I figured I'd leave that up to Thierry (and/or anyone else with a vested
> interest), but having mulled it over since that previous thread, there's
> really no excuse for the API itself not to do the right thing either way, so
> I felt compelled to write up this much.

On Tegra specifically, as the commit message says, we don't really care
about the conditional selection because practically we always want IOMMU
support enabled. So instead of adding back the conditional select it
would make more sense to select IOMMU_API instead and then get rid of
the handful of #ifdef blocks we have for that.

On a side note: I'm looking at a subtle regression right now where some
corner case no longer works and that's primarily due to the fact that we
can have four different scenarios: non-IOMMU, IOMMU, DMA/IOMMU and IOMMU
but devices with no iommus property in their DT nodes. This has all
become very unwieldy and becomes increasingly difficult to test. So
anything that we can do to keep down the number of permutations is
welcome.

While that's not directly related to this issue, the lesson learned is
that, at least on the Tegra side, we have in the past made things
unnecessarily difficult for ourselves based on the misguided assumption
that maximum configurability is key.

Thierry

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