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: <b4d8c5b2-3ad7-4327-9985-d097d095ccb5@samsung.com>
Date: Thu, 12 Jun 2025 08:31:12 +0200
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: Tomi Valkeinen <tomi.valkeinen@...asonboard.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

On 12.06.2025 07:49, Tomi Valkeinen wrote:
> On 11/06/2025 13:45, Marek Szyprowski wrote:
>> 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?

Yes, sorry, my fault. I much have checked the same (peach-pi) dts 2 
times. Both Peach-Pi and Peach-Pit fails, while Snow works fine. All 
three use the same Exynos DP (based on analogix dp) driver. I will try 
to play a bit more with those boards in the afternoon, hopefully getting 
some more hints where the issue is.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland