linux-kernel - Re: [PATCH 1/1] drm/bridge: Fix handling of bridges with pre_enable_prev

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Date:   Fri, 14 Jul 2023 18:16:47 +0100
From:   Dave Stevenson <dave.stevenson@...pberrypi.com>
To:     Frieder Schrempf <frieder.schrempf@...tron.de>
Cc:     Vladimir Lypak <vladimir.lypak@...il.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Jagan Teki <jagan@...rulasolutions.com>
Subject: Re: [PATCH 1/1] drm/bridge: Fix handling of bridges with
 pre_enable_prev_first flag

Hi Frieder

On Mon, 10 Jul 2023 at 08:46, Frieder Schrempf
<frieder.schrempf@...tron.de> wrote:
>
> On 07.07.23 21:00, Vladimir Lypak wrote:
> > [Sie erhalten nicht häufig E-Mails von vladimir.lypak@...il.com. Weitere Informationen, warum dies wichtig ist, finden Sie unter https://aka.ms/LearnAboutSenderIdentification ]
> >
> > In function drm_atomic_bridge_chain_post_disable handling of
> > pre_enable_prev_first flag is broken because "next" variable will always
> > end up set to value of "bridge". This breaks loop which should disable
> > bridges in reverse:
> >
> >  next = list_next_entry(bridge, chain_node);
> >
> >  if (next->pre_enable_prev_first) {
> >         /* next bridge had requested that prev
> >          * was enabled first, so disabled last
> >          */
> >         limit = 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,
> >                                  chain_node) {
> > // Next condition is always true. It is likely meant to be inversed
> > // according to comment above. But doing this uncovers another problem:
> > // it won't work if there are few bridges with this flag set at the end.
> >                 if (next->pre_enable_prev_first) {
> >                         next = list_prev_entry(next, chain_node);
> >                         limit = next;
> > // Here we always set next = limit = branch at first iteration.
> >                         break;
> >                 }
> >         }
> >
> >         /* Call these bridges in reverse order */
> >         list_for_each_entry_from_reverse(next, &encoder->bridge_chain,
> >                                          chain_node) {
> > // This loop never executes past this branch.
> >                 if (next == bridge)
> >                         break;
> >
> >                 drm_atomic_bridge_call_post_disable(next, old_state);
> >
> > In this patch logic for handling the flag is simplified. Temporary
> > "iter" variable is introduced instead of "next" which is used only
> > inside inner loops.
> >
> > Fixes: 4fb912e5e190 ("drm/bridge: Introduce pre_enable_prev_first to alter bridge init order")
> > Signed-off-by: Vladimir Lypak <vladimir.lypak@...il.com>
>
> I haven't had a chance to look at this, but I still want to reference
> another patch by Jagan that intends to fix some bug in this area:
>
> https://patchwork.kernel.org/project/dri-devel/patch/20230328170752.1102347-1-jagan@amarulasolutions.com/
>
> +Cc: Jagan
>
> Dave, as you introduced this feature, did you have a chance to look at
> Jagan's and Vladimir's patches?

Sorry, they'd fallen off my radar.
I'm having a look at the moment, but will probably need to carry it
over to Monday.

  Dave