linux-kernel - Re: [PATCH v4 02/10] drm/bridge: Fix a use case in the bridge disable logic

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Date:   Wed, 6 Dec 2023 14:26:55 +0100
From:   Dario Binacchi <dario.binacchi@...rulasolutions.com>
To:     Dave Stevenson <dave.stevenson@...pberrypi.com>,
        Jagan Teki <jagan@...rulasolutions.com>
Cc:     linux-kernel@...r.kernel.org,
        Amarula patchwork <linux-amarula@...rulasolutions.com>,
        michael@...rulasolutions.com,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...il.com>,
        Frieder Schrempf <frieder.schrempf@...tron.de>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Jonas Karlman <jonas@...boo.se>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v4 02/10] drm/bridge: Fix a use case in the bridge disable logic

Hi Dave and Jagan,

On Tue, Dec 5, 2023 at 4:39 PM Dave Stevenson
<dave.stevenson@...pberrypi.com> wrote:
>
> Hi Dario
>
> On Tue, 5 Dec 2023 at 10:54, Dario Binacchi
> <dario.binacchi@...rulasolutions.com> wrote:
> >
> > The patch fixes the code for finding the next bridge with the
> > "pre_enable_prev_first" flag set to false. In case this condition is
> > not verified, i. e. there is no subsequent bridge with the flag set to
> > false, the whole bridge list is traversed, invalidating the "next"
> > variable.
> >
> > The use of a new iteration variable (i. e. "iter") ensures that the value
> > of the "next" variable is not invalidated.
>
> We already have https://patchwork.freedesktop.org/patch/529288/ that
> has been reviewed (but not applied) to resolve this. What does this
> version do differently and why?

My patches only affect drm_atomic_bridge_chain_post_disable(), whereas
Jagan's patch affects both
drm_atomic_bridge_chain_post_disable() and drm_atomic_bridge_chain_pre_enable().
I tested Jagan's patch on my system with success and I reviewed with
Michael Trimarchi the
drm_atomic_bridge_chain_pre_enable() fixing and we think it's okay.
We also believe that our changes to post_disable() are better, as we
set the 'next' variable only when required,
and the code is more optimized since the list_is_last() is not called
within the loop.
Would it be possible to use Jagan's patch for fixing
drm_atomic_bridge_chain_pre_enable() and mine for
fixing drm_atomic_bridge_chain_post_disable()?

Thanks and regards,
Dario

>
>   Dave
>
> > Fixes: 4fb912e5e190 ("drm/bridge: Introduce pre_enable_prev_first to alter bridge init order")
> > Co-developed-by: Michael Trimarchi <michael@...rulasolutions.com>
> > Signed-off-by: Michael Trimarchi <michael@...rulasolutions.com>
> > Signed-off-by: Dario Binacchi <dario.binacchi@...rulasolutions.com>
> > ---
> >
> > (no changes since v1)
> >
> >  drivers/gpu/drm/drm_bridge.c | 9 +++++----
> >  1 file changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> > index f66bf4925dd8..2e5781bf192e 100644
> > --- a/drivers/gpu/drm/drm_bridge.c
> > +++ b/drivers/gpu/drm/drm_bridge.c
> > @@ -662,7 +662,7 @@ void drm_atomic_bridge_chain_post_disable(struct drm_bridge *bridge,
> >                                           struct drm_atomic_state *old_state)
> >  {
> >         struct drm_encoder *encoder;
> > -       struct drm_bridge *next, *limit;
> > +       struct drm_bridge *iter, *next, *limit;
> >
> >         if (!bridge)
> >                 return;
> > @@ -680,14 +680,15 @@ void drm_atomic_bridge_chain_post_disable(struct drm_bridge *bridge,
> >                                  * was enabled first, so disabled last
> >                                  */
> >                                 limit = next;
> > +                               iter = next;
> >
> >                                 /* Find the next bridge that has NOT requested
> >                                  * prev to be enabled first / disabled last
> >                                  */
> > -                               list_for_each_entry_from(next, &encoder->bridge_chain,
> > +                               list_for_each_entry_from(iter, &encoder->bridge_chain,
> >                                                          chain_node) {
> > -                                       if (!next->pre_enable_prev_first) {
> > -                                               next = list_prev_entry(next, chain_node);
> > +                                       if (!iter->pre_enable_prev_first) {
> > +                                               next = list_prev_entry(iter, chain_node);
> >                                                 limit = next;
> >                                                 break;
> >                                         }
> > --
> > 2.43.0
> >

-- 

Dario Binacchi

Senior Embedded Linux Developer

dario.binacchi@...rulasolutions.com

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