linux-kernel - Re: [PATCH v6 03/12] drm/bridge: cdns-dsi: Fix phy de-init and flag it so

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Message-ID: <79fc67a5-ac2a-48e2-b3ae-b96b6c29191e@linux.dev>
Date: Mon, 13 Jan 2025 21:09:11 +0530
From: Aradhya Bhatia <aradhya.bhatia@...ux.dev>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
 Laurent Pinchart <laurent.pinchart@...asonboard.com>,
 Andrzej Hajda <andrzej.hajda@...el.com>,
 Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>,
 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>,
 Nishanth Menon <nm@...com>, Vignesh Raghavendra <vigneshr@...com>,
 Devarsh Thakkar <devarsht@...com>, Praneeth Bajjuri <praneeth@...com>,
 Udit Kumar <u-kumar1@...com>, Jayesh Choudhary <j-choudhary@...com>,
 DRI Development List <dri-devel@...ts.freedesktop.org>,
 Linux Kernel List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 03/12] drm/bridge: cdns-dsi: Fix phy de-init and flag
 it so

Thank you for reviewing the patches, Dmitry.

On 1/13/25 15:10, Dmitry Baryshkov wrote:
> On Sun, Jan 12, 2025 at 12:57:29AM +0530, Aradhya Bhatia wrote:
>> From: Aradhya Bhatia <a-bhatia1@...com>
>>
>> The driver code doesn't have a de-initialization path as yet, and so it
>> does not clear the phy_initialized flag while suspending. This is a
>> problem because after resume the driver looks at this flag to determine
>> if a Phy re-initialization is required or not. It is in fact required
>> because the hardware is resuming from a suspend, but the driver does not
>> carry out any re-initialization causing the D-Phy to not work at all.
>>
>> Add the counterpart of phy_power_on(), that is phy_power_off() from the
>> _bridge_disable() and clear the flags so that the Phy can be initialized
>> again when required.
>>
>> Move the Phy initialization from _bridge_enable() to _resume(), and
>> de-initialize during the _suspend() - so that the phy_{init, exit}()
>> take place once every resume/suspend cycle.
> 
> Is it okay to call phy_init() before writing MCTL_DPHY_CFG0 ?

The phy_init() is a no-op when we look at the D-Phy driver, which does
not implement the _init() hook at all. So, in this case, all phy_init()
call ever manages to do is book-keeping. Book-keeping that isn't
required to be done every time we do a bridge enable/disable.

But despite the no-op nature of the call, I guess it would still not
make sense to call it before the reset assert done in MCTL_DPHY_CFG0.

Instead of moving it to resume(), I can keep phy_init() as is, and add
phy_exit() in the bridge disable path, instead of the suspend path.


Regards
Aradhya

> 
>>
>> The order of calls still remains the same. phy_init() needs to be called
>> before phy_power_on() - which happens still. What this patch changes is
>> the frequency of the phy_init() call. Instead of it being called once
>> every bridge enable/disable cycle, it is now being called once every
>> resume/suspend cycle. This move has been considered safe after numerous
>> tests with the hardware.
>>
>> Fixes: fced5a364dee ("drm/bridge: cdns: Convert to phy framework")
>> Signed-off-by: Aradhya Bhatia <a-bhatia1@...com>
>> Signed-off-by: Aradhya Bhatia <aradhya.bhatia@...ux.dev>
>> ---
>>  drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c | 10 ++++++++--
>>  1 file changed, 8 insertions(+), 2 deletions(-)
>