linux-kernel - Re: [PATCH v13 3/4] drm/atomic-helper: Re-order bridge chain pre-enable and post-disable

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Message-ID: <306f142f-f9c9-44ab-a5b9-c71db76b2b80@ideasonboard.com>
Date: Thu, 12 Jun 2025 08:49:04 +0300
From: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To: Marek Szyprowski <m.szyprowski@...sung.com>,
 Aradhya Bhatia <aradhya.bhatia@...ux.dev>
Cc: DRI Development List <dri-devel@...ts.freedesktop.org>,
 Linux Kernel List <linux-kernel@...r.kernel.org>, Nishanth Menon
 <nm@...com>, Vignesh Raghavendra <vigneshr@...com>,
 Devarsh Thakkar <devarsht@...com>, Jayesh Choudhary <j-choudhary@...com>,
 Alexander Sverdlin <alexander.sverdlin@...mens.com>,
 Dmitry Baryshkov <lumag@...nel.org>, 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>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>
Subject: Re: [PATCH v13 3/4] drm/atomic-helper: Re-order bridge chain
 pre-enable and post-disable

Hi,

On 11/06/2025 13:45, Marek Szyprowski wrote:
> Hi,
> 
> On 05.06.2025 19:15, Aradhya Bhatia wrote:
>> From: Aradhya Bhatia <a-bhatia1@...com>
>>
>> Move the bridge pre_enable call before crtc enable, and the bridge
>> post_disable call after the crtc disable.
>>
>> The sequence of enable after this patch will look like:
>>
>> 	bridge[n]_pre_enable
>> 	...
>> 	bridge[1]_pre_enable
>>
>> 	crtc_enable
>> 	encoder_enable
>>
>> 	bridge[1]_enable
>> 	...
>> 	bridge[n]_enable
>>
>> And, the disable sequence for the display pipeline will look like:
>>
>> 	bridge[n]_disable
>> 	...
>> 	bridge[1]_disable
>>
>> 	encoder_disable
>> 	crtc_disable
>>
>> 	bridge[1]_post_disable
>> 	...
>> 	bridge[n]_post_disable
>>
>> The definition of bridge pre_enable hook says that,
>> "The display pipe (i.e. clocks and timing signals) feeding this bridge
>> will not yet be running when this callback is called".
>>
>> Since CRTC is also a source feeding the bridge, it should not be enabled
>> before the bridges in the pipeline are pre_enabled. Fix that by
>> re-ordering the sequence of bridge pre_enable and bridge post_disable.
>>
>> While at it, update the drm bridge API documentation as well.
>>
>> Acked-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
>> Reviewed-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
>> Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>
>> Tested-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
>> Tested-by: Alexander Sverdlin <alexander.sverdlin@...mens.com>
>> Signed-off-by: Aradhya Bhatia <a-bhatia1@...com>
>> Signed-off-by: Aradhya Bhatia <aradhya.bhatia@...ux.dev>
> 
> This patch landed in today's linux-next as commit c9b1150a68d9 
> ("drm/atomic-helper: Re-order bridge chain pre-enable and 
> post-disable"). In my tests I found that it breaks booting of Samsung 
> Exynos 5420/5800 based Chromebooks (Peach-Pit and Peach-Pi). Both of 
> them use Exynos DRM with Exynos_DP sub-driver (Analogix DP) and EDP 
> panel. Booting stops at '[drm] Initialized exynos 1.1.0 for exynos-drm 
> on minor 0' message. On the other hand, the Samsung Exynos5250 based 
> Snow Chromebook boots fine, but it uses dp-lvds nxp,ptn3460 bridge and 
> lvds panel instead of edp panels. This looks like some sort of deadlock, 
> because if I disable FBDEV emulation, those boards boots fine and I'm 
> able to run modetest and enable the display. Also the DRM kernel logger 
> seems to be working fine, although I didn't check the screen output yet, 
> as I only have a remote access to those boards. I will investigate it 
> further and let You know.

Thanks for the report. I was trying to understand the pipeline, but I'm
a bit confused. Above you say Peach-Pit uses DP and EDP panel, but if I
look at arch/arm/boot/dts/samsung/exynos5420-peach-pit.dts, it connects
a dp->lvds bridge (parade,ps8625). Peach-Pi seems to connect to an eDP
panel.

Is the above correct? Do both Peach-Pi and Peach-Pit fail?

 Tomi