linux-kernel - Re: [PATCH] drm: bridge: samsung-dsim: Don't use FORCE_STOP

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Message-ID: <1ef3dad2-5f55-40e5-bba7-3c71d71c12e4@kontron.de>
Date: Mon, 18 Dec 2023 12:24:23 +0100
From: Frieder Schrempf <frieder.schrempf@...tron.de>
To: Michael Walle <mwalle@...nel.org>, Inki Dae <inki.dae@...sung.com>,
 Jagan Teki <jagan@...rulasolutions.com>,
 Marek Szyprowski <m.szyprowski@...sung.com>,
 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>,
 David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
 Tim Harvey <tharvey@...eworks.com>,
 Alexander Stein <alexander.stein@...tq-group.com>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm: bridge: samsung-dsim: Don't use FORCE_STOP_STATE

On 01.12.23 10:04, Michael Walle wrote:
>> The FORCE_STOP_STATE bit is unsuitable to force the DSI link into LP-11
>> mode. It seems the bridge internally queues DSI packets and when the
>> FORCE_STOP_STATE bit is cleared, they are sent in close succession
>> without any useful timing (this also means that the DSI lanes won't go
>> into LP-11 mode). The length of this gibberish varies between 1ms and
>> 5ms. This sometimes breaks an attached bridge (TI SN65DSI84 in this
>> case). In our case, the bridge will fail in about 1 per 500 reboots.
>>
>> The FORCE_STOP_STATE handling was introduced to have the DSI lanes in
>> LP-11 state during the .pre_enable phase. But as it turns out, none of
>> this is needed at all. Between samsung_dsim_init() and
>> samsung_dsim_set_display_enable() the lanes are already in LP-11 mode.
>> The code as it was before commit 20c827683de0 ("drm: bridge:
>> samsung-dsim: Fix init during host transfer") and 0c14d3130654 ("drm:
>> bridge: samsung-dsim: Fix i.MX8M enable flow to meet spec") was correct
>> in this regard.
>>
>> This patch basically reverts both commits. It was tested on an i.MX8M
>> SoC with an SN65DSI84 bridge. The signals were probed and the DSI
>> packets were decoded during initialization and link start-up. After this
>> patch the first DSI packet on the link is a VSYNC packet and the timing
>> is correct.
>>
>> Command mode between .pre_enable and .enable was also briefly tested by
>> a quick hack. There was no DSI link partner which would have responded,
>> but it was made sure the DSI packet was send on the link. As a side
>> note, the command mode seems to just work in HS mode. I couldn't find
>> that the bridge will handle commands in LP mode.
>>
>> Fixes: 20c827683de0 ("drm: bridge: samsung-dsim: Fix init during host
>> transfer")
>> Fixes: 0c14d3130654 ("drm: bridge: samsung-dsim: Fix i.MX8M enable
>> flow to meet spec")
>> Signed-off-by: Michael Walle <mwalle@...nel.org>
>> ---
>> Let me know wether this should be two commits each reverting one, but
>> both
>> commits appeared first in kernel 6.5.
> 
> Are there any news?

Inki, are you picking this up? Or if not, who will?