linux-kernel - Re: [PATCH v6 16/23] drm/probe-helper: Provide a TV get

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Date:   Thu, 27 Oct 2022 00:02:24 +0200
From:   Mateusz Kwiatkowski <kfyatek@...il.com>
To:     maxime@...no.tech, Karol Herbst <kherbst@...hat.com>,
        Emma Anholt <emma@...olt.net>, Ben Skeggs <bskeggs@...hat.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Samuel Holland <samuel@...lland.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Maxime Ripard <mripard@...nel.org>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Lyude Paul <lyude@...hat.com>
Cc:     linux-sunxi@...ts.linux.dev, intel-gfx@...ts.freedesktop.org,
        Phil Elwell <phil@...pberrypi.com>,
        linux-arm-kernel@...ts.infradead.org,
        nouveau@...ts.freedesktop.org, Hans de Goede <hdegoede@...hat.com>,
        Dom Cobley <dom@...pberrypi.com>,
        dri-devel@...ts.freedesktop.org,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        linux-kernel@...r.kernel.org,
        Noralf Trønnes <noralf@...nnes.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v6 16/23] drm/probe-helper: Provide a TV get_modes helper

Hi Maxime,

First of all, nice idea with the helper function that can be reused by different
drivers. This is neat!

But looking at this function, it feels a bit overcomplicated. You're creating
the two modes, then checking which one is the default, then set the preferred
one and possibly reorder them. Maybe it can be simplified somehow?

Although when I tried to refactor it myself, I ended up with something that's
not better at all. Maybe it needs to be complicated, after all :(

Anyway, the current version seems to have a couple of bugs:

> +	if (tv_mode_supported(connector, DRM_MODE_TV_MODE_PAL) ||
> +	    tv_mode_supported(connector, DRM_MODE_TV_MODE_PAL_N) ||
> +	    tv_mode_supported(connector, DRM_MODE_TV_MODE_SECAM)) {
> +		mode = drm_mode_analog_pal_576i(connector->dev);
> +		if (!mode)
> +			return 0;
> +
> +		tv_modes[count++] = mode;
> +	}

If the 480i mode has been created properly, but there's an error creating the
576i one (we enter the if (!mode) clause), the 480i mode will leak.

> +	if (count == 1) {

You're handling the count == 1 case specially, but if count == 0, the rest of
the code will assume that two modes exist and probably segfault in the process.

> +	ret = drm_object_property_get_default_value(&connector->base,
> +						    dev->mode_config.tv_mode_property,
> +						    &default_mode);
> +	if (ret)
> +		return 0;
> +
> +	if (cmdline->tv_mode_specified)
> +		default_mode = cmdline->tv_mode;

In case of an error (ret != 0), the modes created so far in the tv_modes array
will leak.

Also, I wonder if maybe the if (cmdline->tv_mode_specified) clause should go
first? If we're going to use the default from cmdline, there's no point in even
querying the property default value.

Best regards,
Mateusz Kwiatkowski