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

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Date:   Mon, 7 Nov 2022 13:45:56 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Noralf Trønnes <noralf@...nnes.org>
Cc:     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>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Lyude Paul <lyude@...hat.com>, 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>,
        Mateusz Kwiatkowski <kfyatek+publicgit@...il.com>,
        dri-devel@...ts.freedesktop.org,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        linux-kernel@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v6 16/23] drm/probe-helper: Provide a TV get_modes helper

On Mon, Nov 07, 2022 at 12:29:28PM +0100, Noralf Trønnes wrote:
> 
> 
> Den 07.11.2022 11.21, skrev Maxime Ripard:
> > Hi Noralf,
> > 
> > I'll leave aside your comments on the code, since we'll use your implementation.
> > 
> > On Sun, Nov 06, 2022 at 05:33:48PM +0100, Noralf Trønnes wrote:
> >> Den 26.10.2022 17.33, skrev maxime@...no.tech:
> >>> +
> >>> +	if (cmdline->tv_mode_specified)
> >>> +		default_mode = cmdline->tv_mode;
> >>
> >> I realised that we don't verify tv_mode coming from the command line,
> >> not here and not in the reset helper. Should we do that? A driver should
> >> be programmed defensively to handle an illegal/unsupported value, but it
> >> doesn't feel right to allow an illegal enum value coming through the
> >> core/helpers.
> > 
> > I don't think we can end up with an invalid value here if it's been
> > specified.
> > 
> > We parse the command line through drm_mode_parse_tv_mode() (introduced
> > in patch 13 "drm/modes: Introduce the tv_mode property as a command-line
> > option") that will pick the tv mode part of the command line, and call
> > drm_get_tv_mode_from_name() using it.
> > 
> > drm_get_tv_mode_from_name() will return a EINVAL if it's not a value we
> > expect, and mode->tv_mode is only set on success. And AFAIK, there's no
> > other path that will set tv_mode.
> > 
> 
> I see now that illegal was the wrong word, but if the driver only
> supports ntsc, the user can still set tv_mode=PAL right? And that's an
> unsupported value that the driver can't fulfill, so it errors out. But
> then again maybe that's just how it is, we can also set a display mode
> that the driver can't handle, so this is no different in that respect.
> Yeah, my argument lost some of its strength here :)

I don't think we can handle this better, really. Falling back to NTSC in
that case would really be a stretch: it's a different mode, with a
different TV mode, etc.

It's an even bigger stretch than picking another mode I guess, and like
you said we're not doing that if the mode isn't supported

Maxime

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