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Message-ID: <87y0vkw8ll.fsf@intel.com>
Date: Mon, 28 Apr 2025 19:31:50 +0300
From: Jani Nikula <jani.nikula@...ux.intel.com>
To: Anusha Srivatsa <asrivats@...hat.com>, Neil Armstrong
<neil.armstrong@...aro.org>, Jessica Zhang <quic_jesszhan@...cinc.com>,
Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
<mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, David
Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, Luca
Ceresoli <luca.ceresoli@...tlin.com>, Anusha Srivatsa
<asrivats@...hat.com>
Subject: Re: [PATCH v4 2/4] drm/panel: Add refcount support
On Mon, 31 Mar 2025, Anusha Srivatsa <asrivats@...hat.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.ceresoli@...tlin.com>
> Reviewed-by: Maxime Ripard <mripard@...nel.org>
> Signed-off-by: Anusha Srivatsa <asrivats@...hat.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. 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.
BR,
Jani.
> +
> + err = devm_add_action_or_reset(dev, drm_panel_put_void, panel);
> + if (err)
> + return ERR_PTR(err);
>
> drm_panel_init(panel, dev, funcs, connector_type);
>
> diff --git a/include/drm/drm_panel.h b/include/drm/drm_panel.h
> index 415e85e8b76a15679f59c944ea152367dc3e0488..31d84f901c514c93ab6cbc09f445853ef897bc95 100644
> --- a/include/drm/drm_panel.h
> +++ b/include/drm/drm_panel.h
> @@ -28,6 +28,7 @@
> #include <linux/errno.h>
> #include <linux/list.h>
> #include <linux/mutex.h>
> +#include <linux/kref.h>
>
> struct backlight_device;
> struct dentry;
> @@ -266,6 +267,17 @@ struct drm_panel {
> * If true then the panel has been enabled.
> */
> bool enabled;
> +
> + /**
> + * @container: Pointer to the private driver struct embedding this
> + * @struct drm_panel.
> + */
> + void *container;
> +
> + /**
> + * @refcount: reference count of users referencing this panel.
> + */
> + struct kref refcount;
> };
>
> void *__devm_drm_panel_alloc(struct device *dev, size_t size, size_t offset,
> @@ -282,6 +294,10 @@ void *__devm_drm_panel_alloc(struct device *dev, size_t size, size_t offset,
> * @connector_type: the connector type (DRM_MODE_CONNECTOR_*) corresponding to
> * the panel interface
> *
> + * The reference count of the returned panel is initialized to 1. This
> + * reference will be automatically dropped via devm (by calling
> + * drm_panel_put()) when @dev is removed.
> + *
> * Returns:
> * Pointer to container structure embedding the panel, ERR_PTR on failure.
> */
> @@ -294,6 +310,9 @@ void drm_panel_init(struct drm_panel *panel, struct device *dev,
> const struct drm_panel_funcs *funcs,
> int connector_type);
>
> +struct drm_panel *drm_panel_get(struct drm_panel *panel);
> +void drm_panel_put(struct drm_panel *panel);
> +
> void drm_panel_add(struct drm_panel *panel);
> void drm_panel_remove(struct drm_panel *panel);
--
Jani Nikula, Intel
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