linux-kernel - Re: [PATCH v5 12/22] drm/connector: Add a function to lookup a TV mode by its name

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Date:   Thu, 20 Oct 2022 13:29:00 +0200
From:   maxime@...no.tech
To:     Noralf Trønnes <noralf@...nnes.org>
Cc:     Karol Herbst <kherbst@...hat.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Lyude Paul <lyude@...hat.com>, Emma Anholt <emma@...olt.net>,
        Chen-Yu Tsai <wens@...e.org>,
        Samuel Holland <samuel@...lland.org>,
        Ben Skeggs <bskeggs@...hat.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Dom Cobley <dom@...pberrypi.com>, linux-sunxi@...ts.linux.dev,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        nouveau@...ts.freedesktop.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-arm-kernel@...ts.infradead.org,
        Mateusz Kwiatkowski <kfyatek+publicgit@...il.com>,
        dri-devel@...ts.freedesktop.org,
        Hans de Goede <hdegoede@...hat.com>,
        Phil Elwell <phil@...pberrypi.com>
Subject: Re: [PATCH v5 12/22] drm/connector: Add a function to lookup a TV
 mode by its name

Hi Noralf,

On Wed, Oct 19, 2022 at 12:43:19PM +0200, Noralf Trønnes wrote:
> 
> 
> Den 19.10.2022 10.48, skrev Maxime Ripard:
> > On Tue, Oct 18, 2022 at 02:29:00PM +0200, Noralf Trønnes wrote:
> >>
> >>
> >> Den 18.10.2022 11.33, skrev Maxime Ripard:
> >>> On Mon, Oct 17, 2022 at 12:44:45PM +0200, Noralf Trønnes wrote:
> >>>> Den 13.10.2022 15.18, skrev Maxime Ripard:
> >>>>> As part of the command line parsing rework coming in the next patches,
> >>>>> we'll need to lookup drm_connector_tv_mode values by their name, already
> >>>>> defined in drm_tv_mode_enum_list.
> >>>>>
> >>>>> In order to avoid any code duplication, let's do a function that will
> >>>>> perform a lookup of a TV mode name and return its value.
> >>>>>
> >>>>> Signed-off-by: Maxime Ripard <maxime@...no.tech>
> >>>>> ---
> >>>>>  drivers/gpu/drm/drm_connector.c | 24 ++++++++++++++++++++++++
> >>>>>  include/drm/drm_connector.h     |  2 ++
> >>>>>  2 files changed, 26 insertions(+)
> >>>>>
> >>>>> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> >>>>> index 820f4c730b38..30611c616435 100644
> >>>>> --- a/drivers/gpu/drm/drm_connector.c
> >>>>> +++ b/drivers/gpu/drm/drm_connector.c
> >>>>> @@ -991,6 +991,30 @@ static const struct drm_prop_enum_list drm_tv_mode_enum_list[] = {
> >>>>>  };
> >>>>>  DRM_ENUM_NAME_FN(drm_get_tv_mode_name, drm_tv_mode_enum_list)
> >>>>>  
> >>>>> +/**
> >>>>> + * drm_get_tv_mode_from_name - Translates a TV mode name into its enum value
> >>>>> + * @name: TV Mode name we want to convert
> >>>>> + * @len: Length of @name
> >>>>> + *
> >>>>> + * Translates @name into an enum drm_connector_tv_mode.
> >>>>> + *
> >>>>> + * Returns: the enum value on success, a negative errno otherwise.
> >>>>> + */
> >>>>> +int drm_get_tv_mode_from_name(const char *name, size_t len)
> >>>>
> >>>> Do we really need to pass in length here? item->name has to always be
> >>>> NUL terminated otherwise things would break elsewhere, so it shouldn't
> >>>> be necessary AFAICS.
> >>>
> >>> The only user so far is the command-line parsing code, and we might very
> >>> well have an option after the tv_mode, something like
> >>> 720x480i,tv_mode=NTSC,rotate=180
> >>>
> >>> In this case, we won't get a NULL-terminated name.
> >>
> >> My point is that item->name will always be NUL terminated so strcmp()
> >> will never look past that.
> > 
> > Right, but we don't have the guarantee that strlen(item->name) <
> > strlen(name), and we could thus just access after the end of our name
> > 
> 
> Ok, using the length limiting str funtions is the safe thing to do, so
> len needs to stay. But I don't get the 'strlen(item->name) == len'
> check. strncmp() will stop when it reaches a NUL in either string so no
> need for the length check?

Yeah, but if the cmdline is truncated, we'll pass a shorter len than
strlen(item->name), and it will consider the string as equal.

For example strncmp("NTS", "NTSC", strlen("NTS"))) == 0, while it obviously
isn't for us.

> Anyways:
> 
> Reviewed-by: Noralf Trønnes <noralf@...nnes.org>

Thanks!
Maxime

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