Re: [PATCH v7 05/11] drm/bridge: add a cleanup action for scope-based drm_bridge_put() invocation

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

On Fri, 14 Mar 2025 19:08:22 +0100
Maxime Ripard <mripard@kernel.org> wrote:

> On Fri, Mar 14, 2025 at 11:31:18AM +0100, Luca Ceresoli wrote:
> > Many functions get a drm_bridge pointer, only use it in the function body
> > (or a smaller scope such as a loop body), and don't store it. In these
> > cases they always need to drm_bridge_put() it before returning (or exiting
> > the scope).
> > 
> > Some of those functions have complex code paths with multiple return points
> > or loop break/continue. This makes adding drm_bridge_put() in the right
> > places tricky, ugly and error prone in case of future code changes.
> > 
> > Others use the bridge pointer in the return statement and would need to
> > split the return line to fit the drm_bridge_put, which is a bit annoying:
> > 
> >   -return some_thing(bridge);
> >   +ret = some_thing(bridge);
> >   +drm_bridge_put(bridge);
> >   +return ret;
> > 
> > To make it easier for all of them to put the bridge reference correctly
> > without complicating code, define a scope-based cleanup action to be used
> > with __free().
> > 
> > Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> > 
> > ---
> > 
> > This patch was added in v7.
> > ---
> >  include/drm/drm_bridge.h | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
> > index 5c1e2b9cafb12eb429d1f5d3ef312e6cf9b54f47..a5accd64c364ebb57903ae1e7459034ad9ebf4f3 100644
> > --- a/include/drm/drm_bridge.h
> > +++ b/include/drm/drm_bridge.h
> > @@ -23,6 +23,7 @@
> >  #ifndef __DRM_BRIDGE_H__
> >  #define __DRM_BRIDGE_H__
> >  
> > +#include <linux/cleanup.h>
> >  #include <linux/ctype.h>
> >  #include <linux/list.h>
> >  #include <linux/mutex.h>
> > @@ -995,6 +996,9 @@ static inline struct drm_bridge *drm_bridge_put(struct drm_bridge *bridge)
> >  	return bridge;
> >  }
> >  
> > +/* Cleanup action for use with __free() */
> > +DEFINE_FREE(drm_bridge_put, struct drm_bridge *, if (_T) drm_bridge_put(_T))
> > +  
> 
> IIRC, drm_bridge_put already checks for the pointer being null before
> dereferencing it, right? Then we can probably simplify the macro here.

drm_bridge_put() does the NULL-check, yes. However I have kept the 'if'
here for consistency with all other DEFINE_FREE()s in the kernel which
also have an 'if' even when the called function does it as well.

Moreover, as per discussion after patch 2, in the next iteration I'm
moving drm_bridge_put() back to a an exported symbol instead of an
inline. So the 'if' here will save a useless function call on NULL.

For the two above reasons I'm rather inclined to keeping this line as
is.

> Either way,
> 
> Reviewed-by: Maxime Ripard <mripard@kernel.org>

Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com