drm: Add docs for managed resources

All collected together to provide a consistent story in one patch,
instead of the somewhat bumpy refactor-evolution leading to this.

Also some thoughts on what the next steps could be:

- Create a macro called devm_drm_dev_alloc() which essentially wraps
  the kzalloc(); devm_drm_dev_init(); drmm_add_final_kfree() combo.
  Needs to be a macro since we'll have to do some typeof trickery and
  casting to make this fully generic for all drivers that embed struct
  drm_device into their own thing.

- A lot of the simple drivers now have essentially just
  drm_dev_unplug(); drm_atomic_helper_shutdown(); as their
  $bus_driver->remove hook. We could create a devm_mode_config_reset
  which sets drm_atomic_helper_shutdown as it's cleanup action, and a
  devm_drm_dev_register with drm_dev_unplug as it's cleanup action,
  and simple drivers wouldn't have a need for a ->remove function at
  all, and we could delete them.

- For more complicated drivers we need drmm_ versions of a _lot_ more
  things. All the userspace visible objects (crtc, plane, encoder,
  crtc), anything else hanging of those (maybe a drmm_get_edid, at
  least for panels and other built-in stuff).

Also some more thoughts on why we're not reusing devm_ with maybe a
fake struct device embedded into the drm_device (we can't use the
kdev, since that's in each drm_minor).

- Code review gets extremely tricky, since every time you see a devm_
  you need to carefully check whether the fake device (with the
  drm_device lifetim) or the real device (with the lifetim of the
  underlying physical device and driver binding) are used. That's not
  going to help at all, and we have enormous amounts of drivers who
  use devm_ where they really shouldn't. Having different types makes
  sure the compiler type checks this for us and ensures correctness.

- The set of functions are very much non-overlapping. E.g.
  devm_ioremap makes total sense, drmm_ioremap has the wrong lifetime,
  since hw resources need to be cleaned out at driver unbind and wont
  outlive that like a drm_device. Similar, but other way round for
  drmm_connector_init (which is the only correct version, devm_ for
  drm_connector is just buggy). Simply not having the wrong version
  again prevents bugs.

Finally I guess this opens a huge todo for all the drivers. I'm
semi-tempted to do a tree-wide s/devm_kzalloc/drmm_kzalloc/ since most
likely that'll fix an enormous amount of bugs and most likely not
cause any issues at all (aside from maybe holding onto memory slightly
too long).

v2:
- Doc improvements from Laurent.
- Also add kerneldoc for the new drmm_add_action_or_reset.

v3:
- Remove kerneldoc for drmm_remove_action.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

fixup docs
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-52-daniel.vetter@ffwll.ch

2 files changed, 59 insertions, 0 deletions

diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h
index b778f3642a90..e0ea577559ff 100644
--- a/include/drm/drm_drv.h
+++ b/include/drm/drm_drv.h
@@ -263,6 +263,10 @@ struct drm_driver {
 	 *
 	 * Optional callback for destroying device data after the final
 	 * reference is released, i.e. the device is being destroyed.
+	 *
+	 * This is deprecated, clean up all memory allocations associated with a
+	 * &drm_device using drmm_add_action(), drmm_kmalloc() and related
+	 * managed resources functions.
 	 */
 	void (*release) (struct drm_device *);
 
diff --git a/include/drm/drm_managed.h b/include/drm/drm_managed.h
index 191d8d206ff4..ca4114633bf9 100644
--- a/include/drm/drm_managed.h
+++ b/include/drm/drm_managed.h
@@ -11,6 +11,16 @@ struct drm_device;
 
 typedef void (*drmres_release_t)(struct drm_device *dev, void *res);
 
+/**
+ * drmm_add_action - add a managed release action to a &drm_device
+ * @dev: DRM device
+ * @action: function which should be called when @dev is released
+ * @data: opaque pointer, passed to @action
+ *
+ * This function adds the @release action with optional parameter @data to the
+ * list of cleanup actions for @dev. The cleanup actions will be run in reverse
+ * order in the final drm_dev_put() call for @dev.
+ */
 #define drmm_add_action(dev, action, data) \
 	__drmm_add_action(dev, action, data, #action)
 
@@ -18,6 +28,15 @@ int __must_check __drmm_add_action(struct drm_device *dev,
 				   drmres_release_t action,
 				   void *data, const char *name);
 
+/**
+ * drmm_add_action_or_reset - add a managed release action to a &drm_device
+ * @dev: DRM device
+ * @action: function which should be called when @dev is released
+ * @data: opaque pointer, passed to @action
+ *
+ * Similar to drmm_add_action(), with the only difference that upon failure
+ * @action is directly called for any cleanup work necessary on failures.
+ */
 #define drmm_add_action_or_reset(dev, action, data) \
 	__drmm_add_action_or_reset(dev, action, data, #action)
 
@@ -28,10 +47,33 @@ int __must_check __drmm_add_action_or_reset(struct drm_device *dev,
 void drmm_add_final_kfree(struct drm_device *dev, void *container);
 
 void *drmm_kmalloc(struct drm_device *dev, size_t size, gfp_t gfp) __malloc;
+
+/**
+ * drmm_kzalloc - &drm_device managed kzalloc()
+ * @dev: DRM device
+ * @size: size of the memory allocation
+ * @gfp: GFP allocation flags
+ *
+ * This is a &drm_device managed version of kzalloc(). The allocated memory is
+ * automatically freed on the final drm_dev_put(). Memory can also be freed
+ * before the final drm_dev_put() by calling drmm_kfree().
+ */
 static inline void *drmm_kzalloc(struct drm_device *dev, size_t size, gfp_t gfp)
 {
 	return drmm_kmalloc(dev, size, gfp | __GFP_ZERO);
 }
+
+/**
+ * drmm_kmalloc_array - &drm_device managed kmalloc_array()
+ * @dev: DRM device
+ * @n: number of array elements to allocate
+ * @size: size of array member
+ * @flags: GFP allocation flags
+ *
+ * This is a &drm_device managed version of kmalloc_array(). The allocated
+ * memory is automatically freed on the final drm_dev_put() and works exactly
+ * like a memory allocation obtained by drmm_kmalloc().
+ */
 static inline void *drmm_kmalloc_array(struct drm_device *dev,
 				       size_t n, size_t size, gfp_t flags)
 {
@@ -42,11 +84,24 @@ static inline void *drmm_kmalloc_array(struct drm_device *dev,
 
 	return drmm_kmalloc(dev, bytes, flags);
 }
+
+/**
+ * drmm_kcalloc - &drm_device managed kcalloc()
+ * @dev: DRM device
+ * @n: number of array elements to allocate
+ * @size: size of array member
+ * @flags: GFP allocation flags
+ *
+ * This is a &drm_device managed version of kcalloc(). The allocated memory is
+ * automatically freed on the final drm_dev_put() and works exactly like a
+ * memory allocation obtained by drmm_kmalloc().
+ */
 static inline void *drmm_kcalloc(struct drm_device *dev,
 				 size_t n, size_t size, gfp_t flags)
 {
 	return drmm_kmalloc_array(dev, n, size, flags | __GFP_ZERO);
 }
+
 char *drmm_kstrdup(struct drm_device *dev, const char *s, gfp_t gfp);
 
 void drmm_kfree(struct drm_device *dev, void *data);