Message-ID: <a3973fac-7ed9-444e-864b-5cfabf8f795f@ideasonboard.com>
Date: Mon, 11 Aug 2025 10:56:19 +0300
From: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To: devarsh <devarsht@...com>, Louis Chauvet <louis.chauvet@...tlin.com>
Cc: thomas.petazzoni@...tlin.com, Jyri Sarha <jsarha@...com>,
 Tomi Valkeinen <tomi.valkeinen@...com>, dri-devel@...ts.freedesktop.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, stable@...r.kernel.org,
 Jyri Sarha <jyri.sarha@....fi>,
 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>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, Sam Ravnborg <sam@...nborg.org>,
 Benoit Parrot <bparrot@...com>, Lee Jones <lee@...nel.org>,
 Nishanth Menon <nm@...com>, Vignesh Raghavendra <vigneshr@...com>,
 Tero Kristo <kristo@...nel.org>, s-jain1@...com
Subject: Re: [PATCH 0/4] drm/tidss: Fixes data edge sampling

Hi,

On 08/08/2025 16:24, devarsh wrote:
> Hi Tomi, Louis,
> 
> On 07/08/25 18:51, Tomi Valkeinen wrote:
>> Hi,
>>
>> On 30/07/2025 20:02, Louis Chauvet wrote:
>>> Currently the driver only configure the data edge sampling partially. The 
>>> AM62 require it to be configured in two distincts registers: one in tidss 
>>> and one in the general device registers.
>>>
>>> Introduce a new dt property to link the proper syscon node from the main 
>>> device registers into the tidss driver.
>>>
>>> Fixes: 32a1795f57ee ("drm/tidss: New driver for TI Keystone platform Display SubSystem")
>>> ---
>>> Cc: stable@...r.kernel.org
>>>
>>> Signed-off-by: Louis Chauvet <louis.chauvet@...tlin.com>
>>
>> I understand why you call this a fix, but I think this is not really a
>> fix. From looking at the patches, my understanding is that for DPI
>> outputs we have always only supported certain clock/data edge.
> 
> I don't think driver makes a distinction between supported/unsupported
> or errors out in case it is run with "different" clock/data edge panel
> (for e.g  DRM_BUS_FLAG_PIXDATA_DRIVE_NEGEDGE is set per the panel
> configuration). Instead it tries to program the VP registers per the
> DRM_BUS_FLAG* getting passed by framework per the connected panel and
> gives an incorrect behavior if those are different than defaults since
> those settings are not sufficient for these displays and instead extra
> MMR register settings are also required.

Well, this gets into the meaning of "fix". I didn't right away see an
explicit definition in the kernel docs.

When the tidss driver was added, neither AM62x nor AM62A existed. Yet
this series "fixes" the original tidss commit for AM62x and AM62A? And
the patch proposes that this series is to be backported to stable
kernels going back to the original tidss commit?

When AM62x and AM62A support was added to the tidss, this feature was
not in the driver. So this is clearly also not a regression. Missing
this feature causes no crashes or other system level misbehavior. It
only causes the panels (that have never been supported with tidss on
AM62x and AM62A) to show garbage.

So we have a driver, to which support for new SoCs was added at some
later point, and at that point we did not add support for all kinds of
panels. Is adding support for those panels a new feature or a bug fix?
Should it be backported to stable kernels?

Documentation/process/stable-kernel-rules.rst has some guidelines. Maybe
one could argue that this is a "hardware quirk" mentioned there, or
perhaps "add a device ID" (of sorts). I might agree, if this was an
easily backportable, totally non-controversial, one-liner style patch
with no chance of regressions. Maybe the next version will be, but this
one is not.

 Tomi

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