On Thu, May 8, 2025 at 10:27 AM Jani Nikula <jani.nik...@linux.intel.com>
wrote:
> On Mon, 05 May 2025, Anusha Srivatsa <asriv...@redhat.com> wrote:
> > On Mon, May 5, 2025 at 2:54 AM Maxime Ripard <mrip...@kernel.org> wrote:
> >
> >> Hi Jani,
> >>
> >> On Tue, Apr 29, 2025 at 12:22:00PM +0300, Jani Nikula wrote:
> >> > On Tue, 29 Apr 2025, Maxime Ripard <mrip...@kernel.org> wrote:
> >> > > Hi Jani,
> >> > >
> >> > > On Mon, Apr 28, 2025 at 07:31:50PM +0300, Jani Nikula wrote:
> >> > >> On Mon, 31 Mar 2025, Anusha Srivatsa <asriv...@redhat.com> wrote:
> >> > >> > Allocate panel via reference counting. Add _get() and _put()
> helper
> >> > >> > functions to ensure panel allocations are refcounted. Avoid use
> >> after
> >> > >> > free by ensuring panel pointer is valid and can be usable till
> the
> >> last
> >> > >> > reference is put.
> >> > >> >
> >> > >> > Reviewed-by: Luca Ceresoli <luca.ceres...@bootlin.com>
> >> > >> > Reviewed-by: Maxime Ripard <mrip...@kernel.org>
> >> > >> > Signed-off-by: Anusha Srivatsa <asriv...@redhat.com>
> >> > >> >
> >> > >> > ---
> >> > >> > v4: Add refcounting documentation in this patch (Maxime)
> >> > >> >
> >> > >> > v3: Add include in this patch (Luca)
> >> > >> >
> >> > >> > v2: Export drm_panel_put/get() (Maxime)
> >> > >> > - Change commit log with better workding (Luca, Maxime)
> >> > >> > - Change drm_panel_put() to return void (Luca)
> >> > >> > - Code Cleanups - add return in documentation, replace bridge to
> >> > >> > panel (Luca)
> >> > >> > ---
> >> > >> > drivers/gpu/drm/drm_panel.c | 64
> >> ++++++++++++++++++++++++++++++++++++++++++++-
> >> > >> > include/drm/drm_panel.h | 19 ++++++++++++++
> >> > >> > 2 files changed, 82 insertions(+), 1 deletion(-)
> >> > >> >
> >> > >> > diff --git a/drivers/gpu/drm/drm_panel.c
> >> b/drivers/gpu/drm/drm_panel.c
> >> > >> > index
> >>
> bdeab5710ee324dc1742fbc77582250960556308..7b17531d85a4dc3031709919564d2e4d8332f748
> >> 100644
> >> > >> > --- a/drivers/gpu/drm/drm_panel.c
> >> > >> > +++ b/drivers/gpu/drm/drm_panel.c
> >> > >> > @@ -355,24 +355,86 @@ struct drm_panel *of_drm_find_panel(const
> >> struct device_node *np)
> >> > >> > }
> >> > >> > EXPORT_SYMBOL(of_drm_find_panel);
> >> > >> >
> >> > >> > +static void __drm_panel_free(struct kref *kref)
> >> > >> > +{
> >> > >> > + struct drm_panel *panel = container_of(kref, struct
> >> drm_panel, refcount);
> >> > >> > +
> >> > >> > + kfree(panel->container);
> >> > >> > +}
> >> > >> > +
> >> > >> > +/**
> >> > >> > + * drm_panel_get - Acquire a panel reference
> >> > >> > + * @panel: DRM panel
> >> > >> > + *
> >> > >> > + * This function increments the panel's refcount.
> >> > >> > + * Returns:
> >> > >> > + * Pointer to @panel
> >> > >> > + */
> >> > >> > +struct drm_panel *drm_panel_get(struct drm_panel *panel)
> >> > >> > +{
> >> > >> > + if (!panel)
> >> > >> > + return panel;
> >> > >> > +
> >> > >> > + kref_get(&panel->refcount);
> >> > >> > +
> >> > >> > + return panel;
> >> > >> > +}
> >> > >> > +EXPORT_SYMBOL(drm_panel_get);
> >> > >> > +
> >> > >> > +/**
> >> > >> > + * drm_panel_put - Release a panel reference
> >> > >> > + * @panel: DRM panel
> >> > >> > + *
> >> > >> > + * This function decrements the panel's reference count and
> frees
> >> the
> >> > >> > + * object if the reference count drops to zero.
> >> > >> > + */
> >> > >> > +void drm_panel_put(struct drm_panel *panel)
> >> > >> > +{
> >> > >> > + if (panel)
> >> > >> > + kref_put(&panel->refcount, __drm_panel_free);
> >> > >> > +}
> >> > >> > +EXPORT_SYMBOL(drm_panel_put);
> >> > >> > +
> >> > >> > +/**
> >> > >> > + * drm_panel_put_void - wrapper to drm_panel_put() taking a void
> >> pointer
> >> > >> > + *
> >> > >> > + * @data: pointer to @struct drm_panel, cast to a void pointer
> >> > >> > + *
> >> > >> > + * Wrapper of drm_panel_put() to be used when a function taking
> a
> >> void
> >> > >> > + * pointer is needed, for example as a devm action.
> >> > >> > + */
> >> > >> > +static void drm_panel_put_void(void *data)
> >> > >> > +{
> >> > >> > + struct drm_panel *panel = (struct drm_panel *)data;
> >> > >> > +
> >> > >> > + drm_panel_put(panel);
> >> > >> > +}
> >> > >> > +
> >> > >> > void *__devm_drm_panel_alloc(struct device *dev, size_t size,
> >> size_t offset,
> >> > >> > const struct drm_panel_funcs
> *funcs,
> >> > >> > int connector_type)
> >> > >> > {
> >> > >> > void *container;
> >> > >> > struct drm_panel *panel;
> >> > >> > + int err;
> >> > >> >
> >> > >> > if (!funcs) {
> >> > >> > dev_warn(dev, "Missing funcs pointer\n");
> >> > >> > return ERR_PTR(-EINVAL);
> >> > >> > }
> >> > >> >
> >> > >> > - container = devm_kzalloc(dev, size, GFP_KERNEL);
> >> > >> > + container = kzalloc(size, GFP_KERNEL);
> >> > >> > if (!container)
> >> > >> > return ERR_PTR(-ENOMEM);
> >> > >> >
> >> > >> > panel = container + offset;
> >> > >> > + panel->container = container;
> >> > >> > panel->funcs = funcs;
> >> > >> > + kref_init(&panel->refcount);
> >> > >>
> >> > >> Hi Anusha, this should be done in drm_panel_init() instead.
> >> > >>
> >> > >> There are many users of drm_panel that don't use
> >> devm_drm_panel_alloc()
> >> > >> but allocate separately, and call drm_panel_init() only.
> >> > >
> >> > > That wouldn't really work, because then drivers would have allocated
> >> the
> >> > > panel with devm_kzalloc and thus the structure would be freed when
> the
> >> > > device is removed, no matter the reference counting state.
> >> > >
> >> > >> They'll all have refcount set to 0 instead of 1 like kref_init()
> does.
> >> > >>
> >> > >> This means all subsequent get/put pairs on such panels will lead to
> >> > >> __drm_panel_free() being called! But through a lucky coincidence,
> that
> >> > >> will be a nop because panel->container is also not initialized...
> >> > >>
> >> > >> I'm sorry to say, the drm refcounting interface is quite broken for
> >> such
> >> > >> use cases.
> >> > >
> >> > > The plan is to convert all panel drivers to that function, and
> Anusha
> >> > > already sent series to do. It still needs a bit of work, but it
> should
> >> > > land soon-ish.
> >> > >
> >> > > For the transitional period though, it's not clear to me what you
> think
> >> > > is broken at the moment, and / or what should be fixed.
> >> > >
> >> > > Would you prefer an explicit check on container not being 0, with a
> >> > > comment?
> >> >
> >> > I'm looking at what it would take to add drm_panel support to i915 so
> >> > that you could have drm_panel_followers on it. There are gaps of
> course,
> >> > but initially it would mean allocating and freeing drm_panel
> ourselves,
> >> > not via devm_drm_panel_alloc() nor devm_kzalloc(), because none of the
> >> > other stuff is allocated that way. drm_panel would just sit as a
> >> > sub-struct inside struct intel_panel, which is a sub-struct of struct
> >> > intel_connector, which has its own allocation...
> >>
> >> I'm not entirely sure why you would need to allocate it from i915? The
> >> drm_panel structure is only meant to be allocated by panel drivers, and
> >> afaik no panel interface controller is allocating it.
>
> I'm looking into a use case involving drm_panel_follower, which requires
> a drm_panel. I don't really need any of the other stuff in drm_panel.
>
> And basically you'd have one drm_panel per connector that is connected
> to a panel, within the same driver.
>
> >> > But basically in its current state, the refcounting would not be
> >> > reliable for that use case. I guess with panel->container being NULL
> >> > nothing happens, but the idea that the refcount drops back to 0 after
> a
> >> > get/put is a bit scary.
> >> >
> >> > Anyway, I think there should be no harm in moving the kref init to
> >> > drm_panel_init(), right?
> >>
> >> I mean, there is because the plan so far was to remove drm_panel_init()
> :)
>
> The problem with that is that it forces a certain type of allocation of
> drm_panel. devm_drm_panel_alloc() allows embedding drm_panel inside
> another struct, but that's inflexible for our use case, where we'd
> probably like to embed it inside something that's already allocated as
> part of something else.
>
> Jani,
will intel_connector_init() be one of the code points where this will be
inflexible?
Anusha
> I mean devm_drm_panel_alloc() is great, but please don't make its use
> mandatory!
>
> > Jani,
> > the series that converts all drivers to use the new API:
> > https://patchwork.freedesktop.org/series/147082/
> > https://patchwork.freedesktop.org/series/147157/
> > https://patchwork.freedesktop.org/series/147246/
> >
> > not landed yet but these are WIP. Still trying to understand your point
> > though... not sure what is broken.
>
> Nothing upstream is broken per se, but if you allocated drm_panel
> directly yourself, initialized it with drm_panel_init(), its refcount
> would initially be 0, not 1, and each subsequent get/put on it would
> call __drm_panel_free().
>
> Even that doesn't break stuff, because by luck panel->container is also
> NULL in this case.
>
>
> BR,
> Jani.
>
> --
> Jani Nikula, Intel
>
>
