linux-kernel - Re: [RFC 0/3] iommu/samsung: Introduce Exynos sysmmu-v8 driver

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Date:   Fri, 21 Jan 2022 09:35:11 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Sam Protsenko <semen.protsenko@...aro.org>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>
Cc:     Sumit Semwal <sumit.semwal@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Cho KyongHo <pullip.cho@...sung.com>,
        Hyesoo Yu <hyesoo.yu@...sung.com>,
        Janghyuck Kim <janghyuck.kim@...sung.com>,
        Jinkyu Yang <jinkyu1.yang@...sung.com>,
        Alex <acnwigwe@...gle.com>, Carlos Llamas <cmllamas@...gle.com>,
        Daniel Mentz <danielmentz@...gle.com>,
        Erick Reyes <erickreyes@...gle.com>,
        "J . Avila" <elavila@...gle.com>, Jonglin Lee <jonglin@...gle.com>,
        Mark Salyzyn <salyzyn@...gle.com>,
        Thierry Strudel <tstrudel@...gle.com>,
        Will McVicker <willmcvicker@...gle.com>,
        Shawn Guo <shawnguo@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-samsung-soc@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC 0/3] iommu/samsung: Introduce Exynos sysmmu-v8 driver

On 20/01/2022 21:19, Sam Protsenko wrote:
> This is a draft of a new IOMMU driver used in modern Exynos SoCs (like
> Exynos850) and Google's GS101 SoC (used in Pixel 6 phone). Most of its
> code were taken from GS101 downstream kernel [1], with some extra
> patches on top (fixes from Exynos850 downstream kernel and some porting
> changes to adapt it to the mainline kernel). All development history can
> be found at [2].
> 
> Similarities with existing exynos-iommu.c is minimal. I did some
> analysis using similarity-tester tool:
> 
> 8<-------------------------------------------------------------------->8
>     $ sim_c -peu -S exynos-iommu.c "|" samsung-*
> 
>     exynos-iommu.c consists for 15 % of samsung-iommu.c material
>     exynos-iommu.c consists for 1 %  of samsung-iommu-fault.c material
>     exynos-iommu.c consists for 3 %  of samsung-iommu.h material
> 8<-------------------------------------------------------------------->8
> 
> So the similarity is very low, most of that code is some boilerplate
> that shouldn't be extracted to common code (like allocating the memory
> and requesting clocks/interrupts in probe function).

This is not a prove of lack of similarities. The vendor drivers have
proven track of poor quality and a lot of code not compatible with Linux
kernel style.

Therefore comparing mainline driver, reviewed and well tested, with a
vendor out-of-tree driver is wrong. You will almost always have 0% of
similarities, because vendor kernel drivers are mostly developed from
scratch instead of re-using existing drivers.

Recently Samsung admitted it - if I extend existing driver, I will have
to test old and new platform, so it is easier for me to write a new driver.

No, this is not that approach we use it in mainline.

Linaro should know it much better.

> 
> It was tested on v5.4 Android kernel on Exynos850 (E850-96 board) with
> DPU use-case (displaying some graphics to the screen). Also it
> apparently works fine on v5.10 GS101 kernel (on Pixel 6). On mainline
> kernel I managed to build, match and bind the driver. No real world test
> was done, but the changes from v5.10 (where it works fine) are minimal
> (see [2] for details). So I'm pretty sure the driver is functional.

No, we do not take untested code or code for different out-of-tree
kernels, not for mainline.

I am pretty sure drivers is poor or not working.

> 
> For this patch series I'd like to receive some high-level review for
> driver's design and architecture. Coding style and API issues I can fix
> later, when sending real (not RFC) series. Particularly I'd like to hear
> some opinions about:
>   - namings: Kconfig option, file names, module name, compatible, etc
>   - modularity: should this driver be a different platform driver (like
>     in this series), or should it be integrated into existing
>     exynos-iommu.c driver somehow
>   - dt-bindings: does it look ok as it is, or some interface changes are
>     needed

You sent bindings in TXT with dead code inside, and you ask if it is ok.
I consider this approach that you sent whatever junk to us hoping that
we will point all the issues instead of finding them by yourself.

I am pretty sure you have several folks in Linaro who can perform first
review and bring the code closer to mainline style.


Best regards,
Krzysztof