linux-kernel - Re: [PATCH v2 09/41] drm/connector: Add TV standard property

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Message-ID: <30a9d7cd-d9ff-3177-ac6c-e7c1f966d89a@gmail.com>
Date:   Fri, 2 Sep 2022 00:00:33 +0200
From:   Mateusz Kwiatkowski <kfyatek@...il.com>
To:     Maxime Ripard <maxime@...no.tech>,
        Maxime Ripard <mripard@...nel.org>,
        Ben Skeggs <bskeggs@...hat.com>,
        David Airlie <airlied@...ux.ie>, Chen-Yu Tsai <wens@...e.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Lyude Paul <lyude@...hat.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Samuel Holland <samuel@...lland.org>,
        Karol Herbst <kherbst@...hat.com>,
        Noralf Trønnes <noralf@...nnes.org>,
        Emma Anholt <emma@...olt.net>, Daniel Vetter <daniel@...ll.ch>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
Cc:     Hans de Goede <hdegoede@...hat.com>,
        linux-arm-kernel@...ts.infradead.org,
        Phil Elwell <phil@...pberrypi.com>,
        intel-gfx@...ts.freedesktop.org,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        dri-devel@...ts.freedesktop.org, Dom Cobley <dom@...pberrypi.com>,
        linux-kernel@...r.kernel.org, nouveau@...ts.freedesktop.org,
        linux-sunxi@...ts.linux.dev,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v2 09/41] drm/connector: Add TV standard property

Hi Maxime,

W dniu 29.08.2022 o 15:11, Maxime Ripard pisze:
> 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 bitmask 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.

This is not a bitmask property anymore, you've just changed it to an enum.
The commit message is now misleading.

> +static const struct drm_prop_enum_list drm_tv_mode_enum_list[] = {
> +    { DRM_MODE_TV_MODE_NTSC_443, "NTSC-443" },
> +    { DRM_MODE_TV_MODE_NTSC_J, "NTSC-J" },
> +    { DRM_MODE_TV_MODE_NTSC_M, "NTSC-M" },
> +    { DRM_MODE_TV_MODE_PAL_60, "PAL-60" },
> +    { DRM_MODE_TV_MODE_PAL_B, "PAL-B" },
> +    { DRM_MODE_TV_MODE_PAL_D, "PAL-D" },
> +    { DRM_MODE_TV_MODE_PAL_G, "PAL-G" },
> +    { DRM_MODE_TV_MODE_PAL_H, "PAL-H" },
> +    { DRM_MODE_TV_MODE_PAL_I, "PAL-I" },
> +    { DRM_MODE_TV_MODE_PAL_M, "PAL-M" },
> +    { DRM_MODE_TV_MODE_PAL_N, "PAL-N" },
> +    { DRM_MODE_TV_MODE_PAL_NC, "PAL-Nc" },
> +    { DRM_MODE_TV_MODE_SECAM_60, "SECAM-60" },
> +    { DRM_MODE_TV_MODE_SECAM_B, "SECAM-B" },
> +    { DRM_MODE_TV_MODE_SECAM_D, "SECAM-D" },
> +    { DRM_MODE_TV_MODE_SECAM_G, "SECAM-G" },
> +    { DRM_MODE_TV_MODE_SECAM_K, "SECAM-K" },
> +    { DRM_MODE_TV_MODE_SECAM_K1, "SECAM-K1" },
> +    { DRM_MODE_TV_MODE_SECAM_L, "SECAM-L" },
> +};

I did not comment on it the last time, but this list looks a little bit random.

Compared to the standards defined by V4L2, you also define SECAM-60 (a good
thing to define, because why not), but don't define PAL-B1, PAL-D1, PAL-K,
SECAM-H, SECAM-LC (whatever that is - probably just another name for SECAM-L,
see my comment about PAL-Nc below), or NTSC-M-KR (a Korean variant of NTSC).

Like I mentioned previously, I'm personally not a fan of including all those
CCIR/ITU system variants, as they don't mean any difference to the output unless
there is an RF modulator involved. But I get it that they have already been used
and regressing probably wouldn't be a very good idea. But in that case keeping
it consistent with the set of values used by V4L2 would be wise, I think.

> +/**
> + * drm_mode_create_tv_properties - create TV specific connector properties
> + * @dev: DRM device
> + * @supported_tv_modes: Bitmask of TV modes supported (See DRM_MODE_TV_MODE_*)
> +
> + * Called by a driver's TV initialization routine, this function creates
> + * the TV specific connector properties for a given device.  Caller is
> + * responsible for allocating a list of format names and passing them to
> + * this routine.
> + *
> + * Returns:
> + * 0 on success or a negative error code on failure.
> + */
> +int drm_mode_create_tv_properties(struct drm_device *dev,
> +                  unsigned int supported_tv_modes)

supported_tv_modes is supposed to be a bitmask of BIT(DRM_MODE_TV_MODE_*)
(or (1<<DRM_MODE_TV_MODE_*)) rather than DRM_MODE_TV_MODE_* directly, but this
is not said explicitly anywhere in this doc comment.

> +    /**
> +     * @DRM_MODE_TV_MODE_PAL_NC: Seems equivalent to
> +     * @DRM_MODE_TV_MODE_PAL_N.
> +     */
> +    DRM_MODE_TV_MODE_PAL_NC,

AFAIK, the entire reason that "PAL-Nc" is ever mentioned as something separate
from PAL-N is a result of a misunderstanding or misreading of the CCIR/ITU
documents. See also the posting signed as Alchaemist here:
https://en.wikipedia.org/wiki/Talk:PAL#PAL-N_versus_PAL-Nc

That being said, we probably want to keep it if we want to remaing compatible
with the loads of software and drivers which enumerate those as separate
systems. But from a technical standpoint, PAL-N and PAL-Nc (and N/PAL, PAL-CN
etc.) are just different "spellings" referring to exactly the same system.

> +    /**
> +     * @DRM_MODE_TV_MODE_SECAM_K: CCIR System G together with the
> +     * SECAM color system. Similar to @DRM_MODE_TV_MODE_SECAM_G but
> +     * with different channels.
> +     */
> +    DRM_MODE_TV_MODE_SECAM_K,
> +
> +    /**
> +     * @DRM_MODE_TV_MODE_SECAM_K1: CCIR System G together with the
> +     * SECAM color system. Similar to @DRM_MODE_TV_MODE_SECAM_G and
> +     * @DRM_MODE_TV_MODE_SECAM_K but with different channels.
> +     */
> +    DRM_MODE_TV_MODE_SECAM_K1,

Typos: you meant CCIR Systems K and K1, not System G.

Best regards,
Mateusz Kwiatkowski