Re: [PATCH 1/2] drm/panel: Allow powering on panel follower after panel is enabled

Jani Nikula
2025-07-31T10:31:00Z

On Thu, 31 Jul 2025, Pin-Yen Lin <treapking@chromium.org> wrote:
> Some touch controllers have to be powered on after the panel's backlight
> is enabled. To support these controllers, introduce after_panel_enabled
> flag to the panel follower and power on the device after the panel and
> its backlight are enabled.

I think it's *very* confusing and error prone to call follower hooks at
different places depending on a flag. The hook names and documentation
say *when* they get called, and that should not change.

I think the right approach would be to add .panel_enabled and
.panel_disabling hooks to struct drm_panel_follower_funcs, and have them
called after panel (and backlight) have been enabled and before panel
(and backlight) are disabled, respectively.

In i2c-hid-core.c, you'd then have two copies of struct
drm_panel_follower_funcs, and use one or the other depending on the
quirk. You can even reuse the functions.

I think overall it'll be be more consistent, more flexible, and probably
fewer lines of code too.

BR,
Jani.

> Signed-off-by: Pin-Yen Lin <treapking@google.com>

PS. Your Signed-off-by doesn't match the patch author.

> ---
>
> drivers/gpu/drm/drm_panel.c | 47 +++++++++++++++++++++++++++++++++----
> include/drm/drm_panel.h | 8 +++++++
> 2 files changed, 51 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_panel.c b/drivers/gpu/drm/drm_panel.c
> index 650de4da08537..58125f8418f37 100644
> --- a/drivers/gpu/drm/drm_panel.c
> +++ b/drivers/gpu/drm/drm_panel.c
> @@ -133,6 +133,9 @@ void drm_panel_prepare(struct drm_panel *panel)
> panel->prepared = true;
>
> list_for_each_entry(follower, &panel->followers, list) {
> + if (follower->after_panel_enabled)
> + continue;
> +
> ret = follower->funcs->panel_prepared(follower);
> if (ret < 0)
> dev_info(panel->dev, "%ps failed: %d\n",
> @@ -178,6 +181,9 @@ void drm_panel_unprepare(struct drm_panel *panel)
> mutex_lock(&panel->follower_lock);
>
> list_for_each_entry(follower, &panel->followers, list) {
> + if (follower->after_panel_enabled)
> + continue;
> +
> ret = follower->funcs->panel_unpreparing(follower);
> if (ret < 0)
> dev_info(panel->dev, "%ps failed: %d\n",
> @@ -208,6 +214,7 @@ EXPORT_SYMBOL(drm_panel_unprepare);
> */
> void drm_panel_enable(struct drm_panel *panel)
> {
> + struct drm_panel_follower *follower;
> int ret;
>
> if (!panel)
> @@ -218,10 +225,12 @@ void drm_panel_enable(struct drm_panel *panel)
> return;
> }
>
> + mutex_lock(&panel->follower_lock);
> +
> if (panel->funcs && panel->funcs->enable) {
> ret = panel->funcs->enable(panel);
> if (ret < 0)
> - return;
> + goto exit;
> }
> panel->enabled = true;
>
> @@ -229,6 +238,18 @@ void drm_panel_enable(struct drm_panel *panel)
> if (ret < 0)
> DRM_DEV_INFO(panel->dev, "failed to enable backlight: %d\n",
> ret);
> +
> + list_for_each_entry(follower, &panel->followers, list) {
> + if (!follower->after_panel_enabled)
> + continue;
> +
> + ret = follower->funcs->panel_prepared(follower);
> + if (ret < 0)
> + dev_info(panel->dev, "%ps failed: %d\n",
> + follower->funcs->panel_prepared, ret);
> + }
> +exit:
> + mutex_unlock(&panel->follower_lock);
> }
> EXPORT_SYMBOL(drm_panel_enable);
>
> @@ -242,6 +263,7 @@ EXPORT_SYMBOL(drm_panel_enable);
> */
> void drm_panel_disable(struct drm_panel *panel)
> {
> + struct drm_panel_follower *follower;
> int ret;
>
> if (!panel)
> @@ -261,6 +283,18 @@ void drm_panel_disable(struct drm_panel *panel)
> return;
> }
>
> + mutex_lock(&panel->follower_lock);
> +
> + list_for_each_entry(follower, &panel->followers, list) {
> + if (!follower->after_panel_enabled)
> + continue;
> +
> + ret = follower->funcs->panel_unpreparing(follower);
> + if (ret < 0)
> + dev_info(panel->dev, "%ps failed: %d\n",
> + follower->funcs->panel_unpreparing, ret);
> + }
> +
> ret = backlight_disable(panel->backlight);
> if (ret < 0)
> DRM_DEV_INFO(panel->dev, "failed to disable backlight: %d\n",
> @@ -269,9 +303,12 @@ void drm_panel_disable(struct drm_panel *panel)
> if (panel->funcs && panel->funcs->disable) {
> ret = panel->funcs->disable(panel);
> if (ret < 0)
> - return;
> + goto exit;
> }
> panel->enabled = false;
> +
> +exit:
> + mutex_unlock(&panel->follower_lock);
> }
> EXPORT_SYMBOL(drm_panel_disable);
>
> @@ -537,7 +574,8 @@ int drm_panel_add_follower(struct device *follower_dev,
> mutex_lock(&panel->follower_lock);
>
> list_add_tail(&follower->list, &panel->followers);
> - if (panel->prepared) {
> + if ((panel->prepared && !follower->after_panel_enabled) ||
> + (panel->enabled && follower->after_panel_enabled)) {
> ret = follower->funcs->panel_prepared(follower);
> if (ret < 0)
> dev_info(panel->dev, "%ps failed: %d\n",
> @@ -566,7 +604,8 @@ void drm_panel_remove_follower(struct drm_panel_follower *follower)
>
> mutex_lock(&panel->follower_lock);
>
> - if (panel->prepared) {
> + if ((panel->prepared && !follower->after_panel_enabled) ||
> + (panel->enabled && follower->after_panel_enabled)) {
> ret = follower->funcs->panel_unpreparing(follower);
> if (ret < 0)
> dev_info(panel->dev, "%ps failed: %d\n",
> diff --git a/include/drm/drm_panel.h b/include/drm/drm_panel.h
> index 843fb756a2950..aa9b6218702fd 100644
> --- a/include/drm/drm_panel.h
> +++ b/include/drm/drm_panel.h
> @@ -183,6 +183,14 @@ struct drm_panel_follower {
> * The panel we're dependent on; set by drm_panel_add_follower().
> */
> struct drm_panel *panel;
> +
> + /**
> + * @after_panel_enabled
> + *
> + * If true then this panel follower should be powered after the panel
> + * and the backlight are enabled, instead of after panel is prepared.
> + */
> + bool after_panel_enabled;
> };
>
> /**

--
Jani Nikula, Intel