linux-kernel - Re: [PATCH] drm/panel: Fix panel mode type setting logic

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Date:   Wed, 22 Mar 2023 11:49:07 +0100
From:   Neil Armstrong <neil.armstrong@...aro.org>
To:     Doug Anderson <dianders@...omium.org>,
        Jianhua Lu <lujianhua000@...il.com>
Cc:     Sam Ravnborg <sam@...nborg.org>, David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/panel: Fix panel mode type setting logic

Hi,

On 17/03/2023 01:23, Doug Anderson wrote:
> Hi,
> 
> 
> On Tue, Mar 14, 2023 at 4:55 PM Jianhua Lu <lujianhua000@...il.com> wrote:
>>
>> On Tue, Mar 14, 2023 at 10:12:02AM -0700, Doug Anderson wrote:
>>> Hi,
>>>
>>> On Tue, Mar 14, 2023 at 4:45 AM Jianhua Lu <lujianhua000@...il.com> wrote:
>>>>
>>>> Some panels set mode type to DRM_MODE_TYPE_PREFERRED by the number
>>>> of modes. It isn't reasonable, so set the first mode type to
>>>> DRM_MODE_TYPE_PREFERRED. This should be more reasonable.
>>>>
>>>> Signed-off-by: Jianhua Lu <lujianhua000@...il.com>
>>>> ---
>>>>   drivers/gpu/drm/panel/panel-abt-y030xx067a.c     | 2 +-
>>>>   drivers/gpu/drm/panel/panel-auo-a030jtn01.c      | 2 +-
>>>>   drivers/gpu/drm/panel/panel-edp.c                | 4 ++--
>>>>   drivers/gpu/drm/panel/panel-innolux-ej030na.c    | 2 +-
>>>>   drivers/gpu/drm/panel/panel-newvision-nv3051d.c  | 2 +-
>>>>   drivers/gpu/drm/panel/panel-newvision-nv3052c.c  | 2 +-
>>>>   drivers/gpu/drm/panel/panel-novatek-nt35950.c    | 2 +-
>>>>   drivers/gpu/drm/panel/panel-novatek-nt39016.c    | 2 +-
>>>>   drivers/gpu/drm/panel/panel-orisetech-ota5601a.c | 2 +-
>>>>   drivers/gpu/drm/panel/panel-seiko-43wvf1g.c      | 4 ++--
>>>>   drivers/gpu/drm/panel/panel-simple.c             | 4 ++--
>>>>   11 files changed, 14 insertions(+), 14 deletions(-)
>>>
>>> Can you explain more about your motivation here? At least for
>> This demonstrates a bad way to set DRM_MODE_TYPE_PREFERRED for panels
>> with more than one mode. It mislead the future contributors to send
>> a patch with this piece of code. There is also a discussion for it.
>> https://lore.kernel.org/lkml/904bc493-7160-32fd-9709-1dcb978ddbab@linaro.org/
>>> panel-edp and panel-simple it seems like it would be better to leave
>>> the logic alone and manually add DRM_MODE_TYPE_PREFERRED to the right
>>> mode for the rare panel that actually has more than one mode listed.
>> I think we can order it to the first mode if the mode type should be
>> DRM_MODE_TYPE_PREFERRED, It's also same.
> 
> A pointer to the original discussion would have been super helpful to
> be provided in your patch description.
> 
> Personally, I still stand by my assertion that I'd rather that
> DRM_MODE_TYPE_PREFERRED be placed in the actual data instead of being
> done like this in post-processing. To me the old check for "num_modes
> == 1" is basically saying that the people creating the "static const"
> data in this file were lazy and didn't feel like they needed to set a
> DRM_MODE_TYPE_PREFERRED when there was only one mode listed. Thus, we
> can add it for them. When "num_modes" is more than 1 then we shouldn't
> allow the people making the "static const" data to be lazy. We should
> force them to set one of the modes to be PREFERRED rather than for
> them to have to know about this magic rule.
> 
> Thus, for me, my order of preference would be these (note, I've mostly
> looked at panel-edp and panel-simple):
> 
> 1. Leave the check as "num_modes == 1" and document that we're
> basically allowing the people writing the "static const" structure to
> be lazy if there's only one mode. Manually add the
> DRM_MODE_TYPE_PREFERRED flag to the small number of cases where there
> is more than one mode. Possibly add a warning printout if we end up
> without a PREFERRED mode. I'd give a Reviewed-by for this.
> 
> 2. Fully get rid of this logic and add DRM_MODE_TYPE_PREFERRED to all
> of the "static const" data. I guess maybe we can't do that for the
> "timings" in panel-edp and panel-simple. I guess for those I'd be OK
> with just setting PREFERRED on the first timing like your patch is
> doing. I'd give a Reviewed-by for this.
> 
> 3. Your patch. I won't NAK it since it seems like this is what other
> (more senior) DRM folks were suggesting. ...but I don't love it. I
> won't give a Reviewed-by for this but won't object to someone else
> doing so.

I'm aligned with Doug's analysis, I don't have a strong opinion on
what to do, but 1 or 2 would be OK.

Neil

> 
> -Doug