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Date:   Mon, 24 Aug 2020 01:04:35 +0100
From:   Tom Murphy <murphyt7@....ie>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Christoph Hellwig <hch@...radead.org>,
        iommu@...ts.linux-foundation.org, kvm@...r.kernel.org,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Julien Grall <julien.grall@....com>,
        Thierry Reding <thierry.reding@...il.com>,
        Will Deacon <will@...nel.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        linux-samsung-soc@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Jonathan Hunter <jonathanh@...dia.com>,
        linux-rockchip@...ts.infradead.org, Andy Gross <agross@...nel.org>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        linux-s390@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org,
        Alex Williamson <alex.williamson@...hat.com>,
        linux-mediatek@...ts.infradead.org,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        linux-tegra@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        virtualization@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org,
        Robin Murphy <robin.murphy@....com>,
        Cornelia Huck <cohuck@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kukjin Kim <kgene@...nel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
Subject: Re: [PATCH 0/8] Convert the intel iommu driver to the dma-iommu api

Hi Logan/All,

I have added a check for the sg_dma_len == 0 :
"""
 } __sgt_iter(struct scatterlist *sgl, bool dma) {
        struct sgt_iter s = { .sgp = sgl };

+       if (sgl && sg_dma_len(sgl) == 0)
+           s.sgp = NULL;

        if (s.sgp) {
            .....
"""
at location [1].
but it doens't fix the problem.

You're right though, this change does need to be made, this code
doesn't handle pages of sg_dma_len(sg) == 0 correctly
So my guess is that we have more bugs in other parts of the i915
driver (or there is a problem with my "sg_dma_len == 0" fix above).
I have been trying to spot where else the code might be buggy but I
haven't had any luck so far.

I'm doing a microconfernce (at LPC 2020) this wednesdays [1] on this
if you're interested in attending.
I'm hoping I can chat about it with a few people and find how can
reproduce and fix this issues. I don't have any more time I can give
to this unfortunately and it would be a shame for the work to go to
waste.

[0] https://github.com/torvalds/linux/blob/d012a7190fc1fd72ed48911e77ca97ba4521bccd/drivers/gpu/drm/i915/i915_scatterlist.h#L28
[1] https://linuxplumbersconf.org/event/7/contributions/846/

On Fri, 29 May 2020 at 22:21, Logan Gunthorpe <logang@...tatee.com> wrote:
>
>
>
> On 2020-05-29 3:11 p.m., Marek Szyprowski wrote:
> > Patches are pending:
> > https://lore.kernel.org/linux-iommu/20200513132114.6046-1-m.szyprowski@samsung.com/T/
>
> Cool, nice! Though, I still don't think that fixes the issue in
> i915_scatterlist.h given it still ignores sg_dma_len() and strictly
> relies on sg_next()/sg_is_last() to stop iterating -- and I suspect this
> is the bug that got in Tom's way.
>
> >> However, as Robin pointed out, there are other ugly tricks like stopping
> >> iterating through the SGL when sg_dma_len() is zero. For example, the
> >> AMD driver appears to use drm_prime_sg_to_page_addr_arrays() which does
> >> this trick and thus likely isn't buggy (otherwise, I'd expect someone to
> >> have complained by now seeing AMD has already switched to IOMMU-DMA.
> >
> > I'm not sure that this is a trick. Stopping at zero sg_dma_len() was
> > somewhere documented.
>
> Well whatever you want to call it, it is ugly to have some drivers doing
> one thing with the returned value and others assuming there's an extra
> zero at the end. It just causes confusion for people reading/copying the
> code. It would be better if they are all consistent. However, I concede
> stopping at zero should not be broken, presently.
>
> Logan

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