linux-kernel - Re: [PATCH v2 09/33] drm/connector: Add TV standard property

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Date:   Mon, 26 Sep 2022 14:59:33 +0200
From:   Noralf Trønnes <noralf@...nnes.org>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Jernej Skrabec <jernej.skrabec@...il.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Emma Anholt <emma@...olt.net>,
        Karol Herbst <kherbst@...hat.com>,
        Samuel Holland <samuel@...lland.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Daniel Vetter <daniel@...ll.ch>, Lyude Paul <lyude@...hat.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Hans de Goede <hdegoede@...hat.com>,
        nouveau@...ts.freedesktop.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Phil Elwell <phil@...pberrypi.com>,
        intel-gfx@...ts.freedesktop.org, Dom Cobley <dom@...pberrypi.com>,
        linux-sunxi@...ts.linux.dev,
        Mateusz Kwiatkowski <kfyatek+publicgit@...il.com>,
        dri-devel@...ts.freedesktop.org,
        Noralf Trønnes <noralf@...nnes.org>
Subject: Re: [PATCH v2 09/33] drm/connector: Add TV standard property



Den 26.09.2022 12.01, skrev Maxime Ripard:
> On Sat, Sep 24, 2022 at 05:52:29PM +0200, Noralf Trønnes wrote:
>> Den 22.09.2022 16.25, skrev Maxime Ripard:
>>> The TV mode property has been around for a while now to select and get the
>>> current TV mode output on an analog TV connector.
>>>
>>> Despite that property name being generic, its content isn't and has been
>>> driver-specific which makes it hard to build any generic behaviour on top
>>> of it, both in kernel and user-space.
>>>
>>> Let's create a new enum tv norm property, that can contain any of the
>>> analog TV standards currently supported by kernel drivers. Each driver can
>>> then pass in a bitmask of the modes it supports, and the property
>>> creation function will filter out the modes not supported.
>>>
>>> We'll then be able to phase out the older tv mode property.
>>>
>>> Signed-off-by: Maxime Ripard <maxime@...no.tech>
>>
>> Please can you add per patch changelogs, it's hard to review when I have
>> to recall what might have happened with each patch. If you do it drm
>> style and put in the commit message it should be easy enough to do.
> 
> I certainly don't want to start that discussion, but I'm really not a
> fan of that format either. I'll do it for that series if you prefer.
> 

The format isn't important, but especially a big series like this and
being weeks between each iteration it's difficult to follow and see
which review comments that you have chosen to implement and how. It's
almost a full review each time. Even if I see that I have acked/rewieved
a patch, if I don't remember, I have to go back to the previous version
and see if I had any comments and if you followed up on that.

>>> +/**
>>> + * enum drm_connector_tv_mode - Analog TV output mode
>>> + *
>>> + * This enum is used to indicate the TV output mode used on an analog TV
>>> + * connector.
>>> + *
>>> + * WARNING: The values of this enum is uABI since they're exposed in the
>>> + * "TV mode" connector property.
>>> + */
>>> +enum drm_connector_tv_mode {
>>> +	/**
>>> +	 * @DRM_MODE_TV_MODE_NONE: Placeholder to not default on one
>>> +	 * variant or the other when nothing is set.
>>> +	 */
>>> +	DRM_MODE_TV_MODE_NONE = 0,
>>
>> How is this supposed to be used?
> 
> It's not supposed to be used. It was a suggestion from Mateusz to avoid
> to default to any standard when we don't initialize something. I don't
> have any strong feeling about it, so I can drop it if you prefer.
> 

The confusing thing to me is that "None" is part of the property enum
list, so the idea is that it can end up in userspace if there's a driver
error? Hmm, that won't work since TV_MODE_NONE won't be part of the
bitmask that the driver sets. So userspace reading the property ends up
with a value for which there's no enum name to match.

So usespace should be trained to know that zero for this property is a
driver error? No, not a good idea.

I think to catch a bug like this drm_atomic_connector_get_property()
should check the tv.mode value and see if it's a legal enum value and if
not it has to just pick a legal one and print an error. But I'm not sure
it's worth it to catch a bug like this. And I don't see any other enum
properties being checked for validity either before being returned to
userspace.

Based on this reasoning I think you should drop the NONE value.

Noralf.