<div dir="ltr"><div dir="ltr"> </div> <div class="gmail_quote gmail_quote_container"><div dir="ltr" class="gmail_attr">On Mon, May 5, 2025 at 2:54 AM Maxime Ripard <<a href="mailto:mripard@kernel.org">mripard@kernel.org</a>> wrote: </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi Jani, On Tue, Apr 29, 2025 at 12:22:00PM +0300, Jani Nikula wrote: > On Tue, 29 Apr 2025, Maxime Ripard <<a href="mailto:mripard@kernel.org" target="_blank">mripard@kernel.org</a>> wrote: > > Hi Jani, > > > > On Mon, Apr 28, 2025 at 07:31:50PM +0300, Jani Nikula wrote: > >> On Mon, 31 Mar 2025, Anusha Srivatsa <<a href="mailto:asrivats@redhat.com" target="_blank">asrivats@redhat.com</a>> 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 <<a href="mailto:luca.ceresoli@bootlin.com" target="_blank">luca.ceresoli@bootlin.com</a>> > >> > Reviewed-by: Maxime Ripard <<a href="mailto:mripard@kernel.org" target="_blank">mripard@kernel.org</a>> > >> > Signed-off-by: Anusha Srivatsa <<a href="mailto:asrivats@redhat.com" target="_blank">asrivats@redhat.com</a>> > >> > > >> > --- > >> > 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. > 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() :) </blockquote><div> </div><div>Jani,</div><div>the series that converts all drivers to use the new API:</div><div><a href="https://patchwork.freedesktop.org/series/147082/">https://patchwork.freedesktop.org/series/147082/</a></div><div><a href="https://patchwork.freedesktop.org/series/147157/">https://patchwork.freedesktop.org/series/147157/</a></div><div><a href="https://patchwork.freedesktop.org/series/147246/">https://patchwork.freedesktop.org/series/147246/</a></div><div> </div><div>not landed yet but these are WIP. Still trying to understand your point though... not sure what is broken.</div><div> </div><div> </div><div>Anusha</div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> Maxime </blockquote></div></div>