linux-kernel - Re: [PATCH 4/7] drm/msm/dp: fix aux-bus EP lifetime

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Message-ID: <CAD=FV=U8_bjPm3NEOWqzehrx0xFV4U771nTuhhOiM9gKDVCo5g@mail.gmail.com>
Date:   Tue, 13 Sep 2022 07:35:15 +0100
From:   Doug Anderson <dianders@...omium.org>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     Johan Hovold <johan+linaro@...nel.org>,
        Rob Clark <robdclark@...il.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <robert.foss@...aro.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, Sean Paul <sean@...rly.run>,
        Stephen Boyd <swboyd@...omium.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "# 4.0+" <stable@...r.kernel.org>
Subject: Re: [PATCH 4/7] drm/msm/dp: fix aux-bus EP lifetime

Hi,

On Mon, Sep 12, 2022 at 7:10 PM Dmitry Baryshkov
<dmitry.baryshkov@...aro.org> wrote:
>
> On 12/09/2022 18:40, Johan Hovold wrote:
> > Device-managed resources allocated post component bind must be tied to
> > the lifetime of the aggregate DRM device or they will not necessarily be
> > released when binding of the aggregate device is deferred.
> >
> > This can lead resource leaks or failure to bind the aggregate device
> > when binding is later retried and a second attempt to allocate the
> > resources is made.
> >
> > For the DP aux-bus, an attempt to populate the bus a second time will
> > simply fail ("DP AUX EP device already populated").
> >
> > Fix this by amending the DP aux interface and tying the lifetime of the
> > EP device to the DRM device rather than DP controller platform device.
>
> Doug, could you please take a look?
>
> For me this is another reminder/pressure point that we should populate
> the AUX BUS from the probe(), before binding the components together.

Aside from the kernel robot complaints, I'm not necessarily convinced.
I think we know that the AUX DP stuff in MSM-DP is fragile right now
and Qualcomm has promised to clean it up. This really feels like a
band-aid and is really a sign that we're populating the AUX DP bus in
the wrong place in Qualcomm's code. As you said, if we moved this to
probe(), which is the plan in the promised cleanup, then it wouldn't
be a problem.

As far as I know Qualcomm has queued this cleanup behind their current
PSR work (though it's never been clear why both can't be worked on at
the same time) and the PSR work was stalled because they couldn't
figure out what caused the glitching I reported. It's still on my nag
list that I bring up with them every week...

In any case, if a band-aid is urgent, maybe you could just call
of_dp_aux_populate_bus() directly in Qualcomm code and you could add
your own devm_add_action_or_reset() on the DRM device.