linux-kernel - Re: [PATCH 1/3] drm/omap: displays: panel-dpi: add backlight dependency

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Message-ID: <2979804.i6EKSW6ohY@avalon>
Date:   Wed, 19 Apr 2017 23:21:35 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Tomi Valkeinen <tomi.valkeinen@...com>,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] drm/omap: displays: panel-dpi: add backlight dependency

Hi Arnd,

Thank you for the patch.

On Wednesday 19 Apr 2017 19:59:17 Arnd Bergmann wrote:
> The panel driver gained support for backlight but fails to link now
> when that is disabled:
> 
> drivers/gpu/drm/omapdrm/displays/panel-dpi.o: In function
> `panel_dpi_probe_of': panel-dpi.c:(.text.panel_dpi_probe_of+0xe8):
> undefined reference to `of_find_backlight_by_node'
> 
> This adds a dependency like we have for the other panel drivers.

I believe the dependency should be made optional. DPI panels that don't need 
backlight control should be supported by a kernel that has backlight support 
compiled out. How about something like

>From 07a98ab23b2080c79abbf8b5e7479123c50e6be7 Mon Sep 17 00:00:00 2001
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Date: Wed, 19 Apr 2017 23:13:43 +0300
Subject: [PATCH] backlight: Define API stub when backlight support is disabled

The of_find_backlight_by_node() function has a stubbed when CONFIG_OF is
disabled, but drivers that use backlights optionally will still fail to
link if backlight support is disabled. Fix it by defining the stub when
CONFIG_BACKLIGHT_CLASS_DEVICE is disabled.

Signed-off-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>
---
 include/linux/backlight.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/backlight.h b/include/linux/backlight.h
index 5f2fd61ef4fb..fae0b189f7b4 100644
--- a/include/linux/backlight.h
+++ b/include/linux/backlight.h
@@ -162,7 +162,7 @@ struct generic_bl_info {
 	void (*kick_battery)(void);
 };
 
-#ifdef CONFIG_OF
+#if IS_ENABLED(CONFIG_OF) && IS_ENABLED(CONFIG_BACKLIGHT_CLASS_DEVICE)
 struct backlight_device *of_find_backlight_by_node(struct device_node *node);
 #else
 static inline struct backlight_device *


We might need to create stubs for backlight_force_update() and 
backlight_device_set_brightness() too.

> Fixes: 39135a305a0f ("drm/omap: displays: panel-dpi: Support for handling
> backlight devices")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/gpu/drm/omapdrm/displays/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/gpu/drm/omapdrm/displays/Kconfig
> b/drivers/gpu/drm/omapdrm/displays/Kconfig index c226da145fb3..a349cb61961e
> 100644
> --- a/drivers/gpu/drm/omapdrm/displays/Kconfig
> +++ b/drivers/gpu/drm/omapdrm/displays/Kconfig
> @@ -35,6 +35,7 @@ config DRM_OMAP_CONNECTOR_ANALOG_TV
> 
>  config DRM_OMAP_PANEL_DPI
>  	tristate "Generic DPI panel"
> +	depends on BACKLIGHT_CLASS_DEVICE
>  	help
>  	  Driver for generic DPI panels.

-- 
Regards,

Laurent Pinchart