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

1 files changed, 53 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_managed.c b/drivers/gpu/drm/drm_managed.c
index 7246a8318137..4955241ceb4c 100644
--- a/drivers/gpu/drm/drm_managed.c
+++ b/drivers/gpu/drm/drm_managed.c
@@ -20,7 +20,19 @@
  * Inspired by struct &device managed resources, but tied to the lifetime of
  * struct &drm_device, which can outlive the underlying physical device, usually
  * when userspace has some open files and other handles to resources still open.
+ *
+ * Release actions can be added with drmm_add_action(), memory allocations can
+ * be done directly with drmm_kmalloc() and the related functions. Everything
+ * will be released on the final drm_dev_put() in reverse order of how the
+ * release actions have been added and memory has been allocated since driver
+ * loading started with drm_dev_init().
+ *
+ * Note that release actions and managed memory can also be added and removed
+ * during the lifetime of the driver, all the functions are fully concurrent
+ * safe. But it is recommended to use managed resources only for resources that
+ * change rarely, if ever, during the lifetime of the &drm_device instance.
  */
+
 struct drmres_node {
 	struct list_head	entry;
 	drmres_release_t	release;
@@ -111,6 +123,18 @@ static void add_dr(struct drm_device *dev, struct drmres *dr)
 		       dr, dr->node.name, (unsigned long) dr->node.size);
 }
 
+/**
+ * drmm_add_final_kfree - add release action for the final kfree()
+ * @dev: DRM device
+ * @container: pointer to the kmalloc allocation containing @dev
+ *
+ * Since the allocation containing the struct &drm_device must be allocated
+ * before it can be initialized with drm_dev_init() there's no way to allocate
+ * that memory with drmm_kmalloc(). To side-step this chicken-egg problem the
+ * pointer for this final kfree() must be specified by calling this function. It
+ * will be released in the final drm_dev_put() for @dev, after all other release
+ * actions installed through drmm_add_action() have been processed.
+ */
 void drmm_add_final_kfree(struct drm_device *dev, void *container)
 {
 	WARN_ON(dev->managed.final_kfree);
@@ -163,6 +187,16 @@ int __drmm_add_action_or_reset(struct drm_device *dev,
 }
 EXPORT_SYMBOL(__drmm_add_action_or_reset);
 
+/**
+ * drmm_kmalloc - &drm_device managed kmalloc()
+ * @dev: DRM device
+ * @size: size of the memory allocation
+ * @gfp: GFP allocation flags
+ *
+ * This is a &drm_device managed version of kmalloc(). 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().
+ */
 void *drmm_kmalloc(struct drm_device *dev, size_t size, gfp_t gfp)
 {
 	struct drmres *dr;
@@ -181,6 +215,16 @@ void *drmm_kmalloc(struct drm_device *dev, size_t size, gfp_t gfp)
 }
 EXPORT_SYMBOL(drmm_kmalloc);
 
+/**
+ * drmm_kstrdup - &drm_device managed kstrdup()
+ * @dev: DRM device
+ * @s: 0-terminated string to be duplicated
+ * @gfp: GFP allocation flags
+ *
+ * This is a &drm_device managed version of kstrdup(). The allocated memory is
+ * automatically freed on the final drm_dev_put() and works exactly like a
+ * memory allocation obtained by drmm_kmalloc().
+ */
 char *drmm_kstrdup(struct drm_device *dev, const char *s, gfp_t gfp)
 {
 	size_t size;
@@ -197,6 +241,15 @@ char *drmm_kstrdup(struct drm_device *dev, const char *s, gfp_t gfp)
 }
 EXPORT_SYMBOL_GPL(drmm_kstrdup);
 
+/**
+ * drmm_kfree - &drm_device managed kfree()
+ * @dev: DRM device
+ * @data: memory allocation to be freed
+ *
+ * This is a &drm_device managed version of kfree() which can be used to
+ * release memory allocated through drmm_kmalloc() or any of its related
+ * functions before the final drm_dev_put() of @dev.
+ */
 void drmm_kfree(struct drm_device *dev, void *data)
 {
 	struct drmres *dr_match = NULL, *dr;