Re: [PATCH v2 4/8] drm/msm/dpu: Disallow unallocated resources to be returned

[Dmitry Baryshkov]

On 09/01/2023 19:12, Marijn Suijten wrote:
> On 2023-01-09 11:06:45, Dmitry Baryshkov wrote:
> > On Mon, 9 Jan 2023 at 10:24, Marijn Suijten
> > <marijn.suijten@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> > > > > > ---
> > > > > >    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).

Yes. Please excuse me if my original message was not 100% clear.

> - Marijn

--
With best wishes
Dmitry