Re: [PATCH v2 3/5] drm/panel: of_drm_find_panel() return a counted reference

[Albert Esteve]

On Wed, Jul 15, 2026 at 9:14 AM Mikko Perttunen <mperttunen@xxxxxxxxxx> wrote:
>
> On Monday, July 13, 2026 8:53 PM Albert Esteve wrote:
> > Callers of of_drm_find_panel() receive a pointer with no reference
> > held, creating a window where the panel device can be unregistered
> > and freed between the lookup and first use (e.g., drm_panel_prepare()).
> >
> > Fix the lookup function by acquiring a reference with drm_panel_get()
> > before returning, under panel_lock. Callers are now responsible for
> > calling drm_panel_put() when they no longer need the pointer.
> >
> > For bridge drivers that immediately wrap the panel in a panel_bridge
> > (which acquires its own reference), release the lookup reference right
> > after the bridge creation call.
> >
> > For analogix-anx6345, which stores the panel for direct use, release
> > the reference in the i2c remove path.
> >
> > For platform drivers using analogix_dp_core with a component lifecycle
> > (exynos_dp, rockchip analogix_dp), release the lookup reference in the
> > platform remove() function. The panel_bridge created during bind() holds
> > a separate reference that devm cleanup releases after remove() returns.
> >
> > Also fix devm_drm_of_get_bridge() and drmm_of_get_bridge() in
> > bridge/panel.c itself: both call drm_of_find_panel_or_bridge() and
> > then pass the panel to devm/drmm_panel_bridge_add(), which acquires
> > its own reference via drm_panel_bridge_add_typed(). The lookup
> > reference was never released; add drm_panel_put() after each bridge
> > creation call.
> >
> > Assisted-by: Claude:claude-opus-4-6
> > Signed-off-by: Albert Esteve <aesteve@xxxxxxxxxx>
> > ---
> >  drivers/gpu/drm/bridge/analogix/analogix-anx6345.c |  3 +++
> >  drivers/gpu/drm/bridge/panel.c                     |  8 ++++++--
> >  drivers/gpu/drm/drm_of.c                           |  3 ++-
> >  drivers/gpu/drm/drm_panel.c                        | 12 ++++++++----
> >  drivers/gpu/drm/exynos/exynos_dp.c                 | 10 ++++++++++
> >  drivers/gpu/drm/exynos/exynos_drm_dpi.c            |  3 +++
> >  drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_rgb.c          | 18 ++++++++++++++++++
> >  drivers/gpu/drm/logicvc/logicvc_interface.c        | 12 ++++++++++++
> >  drivers/gpu/drm/rockchip/analogix_dp-rockchip.c    | 11 +++++++++++
> >  drivers/gpu/drm/sti/sti_dvo.c                      |  3 +++
> >  drivers/gpu/drm/stm/lvds.c                         |  3 +++
> >  drivers/gpu/drm/sun4i/sun4i_lvds.c                 | 13 +++++++++++++
> >  drivers/gpu/drm/sun4i/sun4i_rgb.c                  | 13 +++++++++++++
> >  drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c             |  2 ++
> >  drivers/gpu/drm/tegra/dsi.c                        |  1 +
> >  drivers/gpu/drm/tegra/output.c                     |  3 +++
> >  16 files changed, 111 insertions(+), 7 deletions(-)
> >
> > ...
> > diff --git a/drivers/gpu/drm/tegra/dsi.c b/drivers/gpu/drm/tegra/dsi.c
> > index 7f25c50621c94..57a016f47434d 100644
> > --- a/drivers/gpu/drm/tegra/dsi.c
> > +++ b/drivers/gpu/drm/tegra/dsi.c
> > @@ -1516,6 +1516,7 @@ static int tegra_dsi_host_detach(struct mipi_dsi_host *host,
> >       struct tegra_output *output = &dsi->output;
> >
> >       if (output->panel && &device->dev == output->panel->dev) {
> > +             drm_panel_put(output->panel);
> >               output->panel = NULL;
> >
> >               if (output->connector.dev)
> > diff --git a/drivers/gpu/drm/tegra/output.c b/drivers/gpu/drm/tegra/output.c
> > index 49e4f63a5550d..90db39dbdd332 100644
> > --- a/drivers/gpu/drm/tegra/output.c
> > +++ b/drivers/gpu/drm/tegra/output.c
> > @@ -195,6 +195,9 @@ int tegra_output_probe(struct tegra_output *output)
> >
> >  void tegra_output_remove(struct tegra_output *output)
> >  {
> > +     if (output->panel)
> > +             drm_panel_put(output->panel);
> > +
> >       if (output->hpd_gpio)
> >               free_irq(output->hpd_irq, output);
> >
> >
>
> tegra_output_probe's error cleanup paths need to be updated to clean up
> the refcount. Also, it can technically call drm_of_find_panel_or_bridge
> and then overwrite that value with of_drm_find_panel. While there's
> already a WARN_ON for that case, I think it would be good to fix the
> refcounting there as well for consistency.

Agreed. I really missed those. I am doing another general check in
case I missed similar cases.

>
> As a note, the tegra-dsi code also uses tegra_output and can overwrite
> the panel field with its own -- but in those cases tegra_output should
> never populate the panel and we've been talking about removing that
> code path anyway, so the dsi code can continue to assume that
> tegra_output_probe doesn't populate a panel.

Thanks. I will add a note before of_drm_find_panel() in tegra/dsi.c to clarify.

BR,
Albert

>
> Cheers
> Mikko
>
> > --
> > 2.54.0
> >
> >
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