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

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Date:   Mon, 10 Jul 2023 09:46:23 +0200
From:   Frieder Schrempf <frieder.schrempf@...tron.de>
To:     Vladimir Lypak <vladimir.lypak@...il.com>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>
Cc:     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

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?