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Date:   Mon, 9 Jan 2023 18:12:23 +0100
From:   Marijn Suijten <marijn.suijten@somainline.org>
To:     Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Cc:     phone-devel@vger.kernel.org, Rob Clark <robdclark@gmail.com>,
        Abhinav Kumar <quic_abhinavk@quicinc.com>,
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        AngeloGioacchino Del Regno 
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        Drew Davenport <ddavenport@chromium.org>
Subject: Re: [PATCH v2 4/8] drm/msm/dpu: Disallow unallocated resources to be
 returned
Message-ID: <20230109171223.vjkigcj7xwfwow2a@SoMainline.org>
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On 2023-01-09 11:06:45, Dmitry Baryshkov wrote:
> On Mon, 9 Jan 2023 at 10:24, Marijn Suijten
> <marijn.suijten@somainline.org> wrote:
> >
> > On 2023-01-09 01:30:29, Dmitry Baryshkov wrote:
> > > On 09/01/2023 01:28, Dmitry Baryshkov wrote:
> > > > On 22/12/2022 01:19, Marijn Suijten wrote:
> > > >> In the event that the topology requests resources that have not been
> > > >> created by the system (because they are typically not represented in
> > > >> dpu_mdss_cfg ^1), the resource(s) in global_state (in this case DSC
> > > >> blocks) remain NULL but will still be returned out of
> > > >> dpu_rm_get_assigned_resources, where the caller expects to get an array
> > > >> containing num_blks valid pointers (but instead gets these NULLs).
> > > >>
> > > >> To prevent this from happening, where null-pointer dereferences
> > > >> typically result in a hard-to-debug platform lockup, num_blks shouldn't
> > > >> increase past NULL blocks and will print an error and break instead.
> > > >> After all, max_blks represents the static size of the maximum number of
> > > >> blocks whereas the actual amount varies per platform.
> > > >>
> > > >> ^1: which can happen after a git rebase ended up moving additions to
> > > >> _dpu_cfg to a different struct which has the same patch context.
> > > >>
> > > >> Fixes: bb00a452d6f7 ("drm/msm/dpu: Refactor resource manager")
> > > >> Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
> > > >> ---
> > > >>   drivers/gpu/drm/msm/disp/dpu1/dpu_rm.c | 5 +++++
> > > >>   1 file changed, 5 insertions(+)
> > > >
> > > > I think the patch is not fully correct. Please check resource
> > > > availability during allocation. I wouldn't expect an error from
> > > > get_assigned_resources because of resource exhaustion.
> >
> > Theoretically patch 5/8 should take care of this, and we should never
> > reach this failure condition.  Emphasis on /should/, this may happen
> > again if/when another block type is added with sub-par resource
> > allocation and assignment implementation.
> 
> Yeah. Maybe swapping 4/8 and 5/8 makes sense.

Ack.

> > > Another option, since allocation functions (except DSC) already have
> > > these safety checks: check error message to mention internal
> > > inconstency: allocated resource doesn't exist.
> >
> > Is this a suggestion for the wording of the error message?
> 
> Yes. Because the current message makes one think that it is output
> during allocation / assignment to encoder, while this is a safety net.

Good.  So the patch is correct, just the wording is off, which I fully
agree on.  This isn't allocating anything, just handing out what was
previously allocated (and is a safety net).

- Marijn