linux-kernel - Re: [RFC PATCH 6/6] drm/msm/dpu: Disallow unallocated (DSC) resources to be returned

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Date:   Wed, 14 Dec 2022 20:31:44 +0100
From:   Marijn Suijten <marijn.suijten@...ainline.org>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     phone-devel@...r.kernel.org, Rob Clark <robdclark@...il.com>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Vinod Koul <vkoul@...nel.org>,
        ~postmarketos/upstreaming@...ts.sr.ht,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...ainline.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Martin Botka <martin.botka@...ainline.org>,
        Jami Kettunen <jami.kettunen@...ainline.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Stephen Boyd <swboyd@...omium.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Jessica Zhang <quic_jesszhan@...cinc.com>,
        Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        Kuogee Hsieh <quic_khsieh@...cinc.com>,
        Jani Nikula <jani.nikula@...el.com>,
        sunliming <sunliming@...inos.cn>,
        Sam Ravnborg <sam@...nborg.org>,
        Haowen Bai <baihaowen@...zu.com>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Loic Poulain <loic.poulain@...aro.org>,
        Vinod Polimera <quic_vpolimer@...cinc.com>,
        Douglas Anderson <dianders@...omium.org>,
        Vladimir Lypak <vladimir.lypak@...il.com>,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 6/6] drm/msm/dpu: Disallow unallocated (DSC)
 resources to be returned

On 2022-12-14 20:56:30, Dmitry Baryshkov wrote:
> On 14/12/2022 01:22, 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.
> > 
> > In the specific case of DSC initial resource allocation should behave
> > more like LMs and CTLs where NULL resources are skipped.  The current
> > hardcoded mapping of DSC blocks should be loosened separately as DPU
> > 5.0.0 introduced a crossbar where DSC blocks can be "somewhat" freely
> > bound to any PP and CTL, but that hardcoding currently means that we
> > will return an error when the topology reserves a DSC that isn't
> > available, instead of looking for the next free one.
> > 
> > ^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.
> > 
> > Signed-off-by: Marijn Suijten <marijn.suijten@...ainline.org>
> > ---
> >   drivers/gpu/drm/msm/disp/dpu1/dpu_rm.c | 10 ++++++++++
> >   1 file changed, 10 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_rm.c b/drivers/gpu/drm/msm/disp/dpu1/dpu_rm.c
> > index 73b3442e7467..dcbf03d2940a 100644
> > --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_rm.c
> > +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_rm.c
> > @@ -496,6 +496,11 @@ static int _dpu_rm_reserve_dsc(struct dpu_rm *rm,
> >   
> >   	/* check if DSC required are allocated or not */
> >   	for (i = 0; i < num_dsc; i++) {
> > +		if (!rm->dsc_blks[i]) {
> > +			DPU_ERROR("DSC %d does not exist\n", i);
> > +			return -EIO;
> > +		}
> > +
> >   		if (global_state->dsc_to_enc_id[i]) {
> >   			DPU_ERROR("DSC %d is already allocated\n", i);
> >   			return -EIO;
> > @@ -660,6 +665,11 @@ int dpu_rm_get_assigned_resources(struct dpu_rm *rm,
> >   				  blks_size, enc_id);
> >   			break;
> >   		}
> > +		if (!hw_blks[i]) {
> > +			DPU_ERROR("No more resource %d available to assign to enc %d\n",
> > +				  type, enc_id);
> > +			break;
> > +		}
> >   		blks[num_blks++] = hw_blks[i];
> >   	}
> >  
> 
> These two chunks should come as two separate patches, each having it's 
> own Fixes tag.

Ack.  They are indeed addressing different issues (with the same
outcome) with differing "backportability".  Will address in v2, thanks
for pointing it out (and missing a Fixes: in the first place, of which
we already have so many...).

- Marijn