linux-kernel - Re: [PATCH 1/2] iommu/exynos: Abstract getting the fault info

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Date:   Wed, 21 Dec 2022 15:32:28 -0600
From:   Sam Protsenko <semen.protsenko@...aro.org>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Janghyuck Kim <janghyuck.kim@...sung.com>,
        Cho KyongHo <pullip.cho@...sung.com>,
        Daniel Mentz <danielmentz@...gle.com>,
        David Virag <virag.david003@...il.com>,
        Sumit Semwal <sumit.semwal@...aro.org>, iommu@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] iommu/exynos: Abstract getting the fault info

Hi Marek,

On Mon, 24 Oct 2022 at 09:43, Sam Protsenko <semen.protsenko@...aro.org> wrote:
>
> Hi Marek,
>
> On Fri, 12 Aug 2022 at 14:25, Marek Szyprowski <m.szyprowski@...sung.com> wrote:
> >
> > Hi Sam,
> >
>
> [snip]
>
> > > Signed-off-by: Sam Protsenko <semen.protsenko@...aro.org>
> >
> > I'm not very happy with converting the sysmmu_fault_info arrays into the
> > decoding functions. If I got the code right, adding v7 is still possible
> > with the current approach. The main advantage of the array-based
> > approach is readability and keeping all the information together in a
> > single place.
> >
> > I agree for the items listed above as 'minor functional changes',
> > though. Those sysmmu_fault_info arrays might be a part of sysmmu hw
> > variant to avoid decoding hw version for each fault.
> >
> > I'm not sure that the linear scan is so problematic with regards to the
> > performance. You really don't want your drivers to trigger IOMMU fault
> > so often during normal operation. It is just a way to get some debugging
> > information or handle some exception.
> >
> > You mentioned that the transaction type is read from the separate
> > register in case of v7, but your code (here and in second patch) still
> > relies on the reported interrupt bits.
> >
> > Could you try to rework all your changes in a such way, that the
> > sysmmu_fault_info arrays are still used? V7 is really very similar to
> > the v5 already supported by the current driver.
> >
>
> That's actually how I implemented this patch on my first attempt.
> Really didn't like it, because a half of existing sysmmu_fault_info
> structure doesn't make sense for v7, and some functionality of v7 has
> to be implemented separately from that structure. I'd argue that
> previous abstraction is just broken, and doesn't work for all SysMMU
> versions anymore. It's easy to see how much difference between v5 and
> v7, just by looking at corresponding get_fault_info() functions I
> implemented. For example, the transaction type is probed from
> different registers using different version, etc. There is also the
> need to handle new VM/non-VM registers on v7. Also there is some extra
> functionality that will be added later, like multiple translation
> domains support, which is also quite different from how things done
> for v5.
>
> I'd show more specifics to demonstrate my statements above, but alas I
> already deleted my initial implementation (which was exactly what you
> suggest). This callback-style HAL seems to be a perfect choice, and I
> spent several days just experimenting with different approaches and
> seeing all pros and cons. And from my point of view, this way is the
> best for providing actual solid abstraction, which doesn't require
> adding any workarounds on top of that. I understand that my patch
> changes the very conception of how IRQ is handled in this driver, but
> I'm still convinced it's a proper way to do that for all v1/v5/v7,
> especially w.r.t. further v7 additions, to keep the abstraction solid.
> Not that I'm lazy and don't want to rework things :) But in this
> particular case I'd go with unchanged patches.
>
> Do you think it's reasonable to take this series as is? I can try and
> collect more particular code snippets to demonstrate my point, if you
> like.
>
> Thanks!
>

So, what do you think about this?

> [snip]