[v9,0/5] drm/bridge: add devm_drm_bridge_alloc() with bridge refcount

Message

Luca Ceresoli March 26, 2025, 5:47 p.m. UTC

This series improves the way DRM bridges are allocated and initialized and
makes them reference-counted. The goal of reference counting is to avoid
use-after-free by drivers which got a pointer to a bridge and keep it
stored and used even after the bridge has been deallocated.

The overall goal is supporting Linux devices with a DRM pipeline whose
final components can be hot-plugged and hot-unplugged, including one or
more bridges. For more details see the big picture [0].

DRM bridge drivers will have to be adapted to the new API -- the change is
trivial for most cases. This series converts two of them to serve as an
example. The remaining ones will be done as a follow-up after this first
part is merged.

After that, refcounting will have to be added on the two sides: all
functions returning a bridge pointer and all code obtaining such a
pointer. A few examples have been sent in v7 (link below), they are OK, but
I removed them since v8 because they must be merged only after converting
all bridges.

Here's the grand plan:

 A. add new alloc API and refcounting (this series, at least patches 1-3)
 B. after (A), convert all bridge drivers to new API
 C. after (A), add documentation and kunit tests
 D. after (B), add get/put to drm_bridge_add/remove() + attach/detech()
    (patches 3-4 in the v7 series)
 E. after (B), convert accessors (including patches 5-9 in the v7 series
    which convert drm_bridge_chain_get_first_bridge() and its users);
    this is a large work and can be done in chunks
 F. after (A), debugfs: show refcount, show removed bridges
 
Layout of this:

 1. Add the new API and refcounting:

    drm/bridge: add devm_drm_bridge_alloc()
    drm/bridge: add support for refcounting
    drm/bridge: deprecate old-style bridge allocation

 2. convert a few bridge drivers (bridge providers) to the new API:

    drm/bridge: ti-sn65dsi83: use dynamic lifetime management
    drm/bridge: samsung-dsim: use dynamic lifetime management

[0] https://lore.kernel.org/dri-devel/[e-mail]/

Signed-off-by: Luca Ceresoli <[e-mail]>
---
Changes in v9 (only patches 2 and 3 have changes):
- Made drm_bridge_put() return void
- Added debugfs improvements to the grand plan (item F)
- Reworded comment in patch 3
- Link to v8: https://lore.kernel.org/r/[e-mail]

Changes in v8:
- Applied requested changes to patch 2
- Add R-by to other patches
- Removed v7 patches 3-9: they are OK but must wait until all bridge
  drivers are converted to the new API
- Added patch to deprecate old-style bridge allocation
- Link to v7: https://lore.kernel.org/r/[e-mail]

---
Luca Ceresoli (5):
      drm/bridge: add devm_drm_bridge_alloc()
      drm/bridge: add support for refcounting
      drm/bridge: make devm_drm_bridge_alloc() mandatory for bridge allocation
      drm/bridge: ti-sn65dsi83: use dynamic lifetime management
      drm/bridge: samsung-dsim: use dynamic lifetime management

 drivers/gpu/drm/bridge/samsung-dsim.c |  7 ++-
 drivers/gpu/drm/bridge/ti-sn65dsi83.c |  7 ++-
 drivers/gpu/drm/drm_bridge.c          | 86 +++++++++++++++++++++++++++++++++++
 include/drm/drm_bridge.h              | 36 +++++++++++++++
 4 files changed, 128 insertions(+), 8 deletions(-)
---
base-commit: 8224bc33fdec82533a68486612e6fd78197b95b5
change-id: 20250314-drm-bridge-refcount-58d9503503f6

Best regards,