On Mon, 29 Sep 2025 14:46:18 +0200 Maxime Ripard <[email protected]> wrote:
> On Fri, Sep 26, 2025 at 05:59:44PM +0200, Luca Ceresoli wrote: > > drm_bridge_attach() modifies the encoder bridge chain, so take a mutex > > around such operations to allow users of the chain to protect themselves > > from chain modifications while iterating. > > > > This change does not apply to drm_bridge_detach() because: > > * only the drm_encoder.c calls it, not bridge drivers (unlike > > drm_bridge_attach()) > > * the only drm_bridge_detach() caller is drm_encoder_cleanup() which > > already locks the mutex for the entire cleanup loop, thus additionally > > locking it here would deadlock > > * drm_bridge_detach() is recursively calling itself along the chain, so > > care would be needed to avoid deadlocks > > Add a comment to clarify that is intended. > > > > Signed-off-by: Luca Ceresoli <[email protected]> > > --- > > drivers/gpu/drm/drm_bridge.c | 9 +++++++++ > > 1 file changed, 9 insertions(+) > > > > diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c > > index > > 630b5e6594e0affad9ba48791207c7b403da5db8..90e467cf91a134342c80d2f958b928472aaf0d8b > > 100644 > > --- a/drivers/gpu/drm/drm_bridge.c > > +++ b/drivers/gpu/drm/drm_bridge.c > > @@ -453,10 +453,12 @@ int drm_bridge_attach(struct drm_encoder *encoder, > > struct drm_bridge *bridge, > > bridge->dev = encoder->dev; > > bridge->encoder = encoder; > > > > + drm_encoder_chain_lock(encoder); > > if (previous) > > list_add(&bridge->chain_node, &previous->chain_node); > > else > > list_add(&bridge->chain_node, &encoder->bridge_chain); > > + drm_encoder_chain_unlock(encoder); > > > > if (bridge->funcs->attach) { > > ret = bridge->funcs->attach(bridge, encoder, flags); > > @@ -487,7 +489,9 @@ int drm_bridge_attach(struct drm_encoder *encoder, > > struct drm_bridge *bridge, > > err_reset_bridge: > > bridge->dev = NULL; > > bridge->encoder = NULL; > > + drm_encoder_chain_lock(encoder); > > list_del(&bridge->chain_node); > > + drm_encoder_chain_unlock(encoder); > > > > if (ret != -EPROBE_DEFER) > > DRM_ERROR("failed to attach bridge %pOF to encoder %s: %d\n", > > @@ -503,6 +507,11 @@ int drm_bridge_attach(struct drm_encoder *encoder, > > struct drm_bridge *bridge, > > } > > EXPORT_SYMBOL(drm_bridge_attach); > > > > +/* > > + * Invoked by the encoder during encoder cleanup in drm_encoder_cleanup(), > > + * so should generally *not* be called by driver code. > > Why not? Because this is what drm_bridge_attach() says O:-) > * drm_bridge_attach - attach the bridge to an encoder's chain ... > * Note that bridges attached to encoders are auto-detached during encoder > * cleanup in drm_encoder_cleanup(), so drm_bridge_attach() should generally > * *not* be balanced with a drm_bridge_detach() in driver code. Also, it's what the code does. > Also, it looks entirely unrelated to the rest of the patch. Sure, I can split it. It is also redundant given that's repeating what drm_bridge_attach() says. I wrote this comment for future people looking at this code. If _attach() takes a lock and _detach() does not, it could look like a potential mistake, and someone could spend precious hours in trying to fix it. Maybe replace with: /* Must be called with the encoder bridge chain locked */ ? Luca -- Luca Ceresoli, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
