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

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Date:   Tue, 5 Dec 2023 15:39:00 +0000
From:   Dave Stevenson <dave.stevenson@...pberrypi.com>
To:     Dario Binacchi <dario.binacchi@...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 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?

  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
>