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From:   maxime@cerno.tech
Date:   Thu, 20 Oct 2022 13:29:00 +0200
To:     Noralf =?utf-8?Q?Tr=C3=B8nnes?= <noralf@tronnes.org>
Cc:     Karol Herbst <kherbst@redhat.com>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
        Daniel Vetter <daniel@ffwll.ch>,
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        David Airlie <airlied@linux.ie>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
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        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        Jernej Skrabec <jernej.skrabec@gmail.com>,
        Dom Cobley <dom@raspberrypi.com>, linux-sunxi@lists.linux.dev,
        Dave Stevenson <dave.stevenson@raspberrypi.com>,
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        Phil Elwell <phil@raspberrypi.com>
Subject: Re: [PATCH v5 12/22] drm/connector: Add a function to lookup a TV
 mode by its name
Message-ID: <20221020112900.dps2zyn4ftefgi3x@houat>
References: <20220728-rpi-analog-tv-properties-v5-0-d841cc64fe4b@cerno.tech>
 <20220728-rpi-analog-tv-properties-v5-12-d841cc64fe4b@cerno.tech>
 <7dcf479c-8ac7-ed47-8587-30268684373c@tronnes.org>
 <20221018093353.pt4vset6o2ldxrbs@houat>
 <e3b98674-5a9e-16f3-4741-ffea43e05cc8@tronnes.org>
 <20221019084828.muy46td63bkyewxk@houat>
 <694facef-00bc-6b59-7c44-d68c7ca0c40f@tronnes.org>
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Hi Noralf,

On Wed, Oct 19, 2022 at 12:43:19PM +0200, Noralf Tr=F8nnes wrote:
>=20
>=20
> Den 19.10.2022 10.48, skrev Maxime Ripard:
> > On Tue, Oct 18, 2022 at 02:29:00PM +0200, Noralf Tr=F8nnes wrote:
> >>
> >>
> >> Den 18.10.2022 11.33, skrev Maxime Ripard:
> >>> On Mon, Oct 17, 2022 at 12:44:45PM +0200, Noralf Tr=F8nnes wrote:
> >>>> Den 13.10.2022 15.18, skrev Maxime Ripard:
> >>>>> As part of the command line parsing rework coming in the next patch=
es,
> >>>>> we'll need to lookup drm_connector_tv_mode values by their name, al=
ready
> >>>>> defined in drm_tv_mode_enum_list.
> >>>>>
> >>>>> In order to avoid any code duplication, let's do a function that wi=
ll
> >>>>> perform a lookup of a TV mode name and return its value.
> >>>>>
> >>>>> Signed-off-by: Maxime Ripard <maxime@cerno.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[] =3D {
> >>>>>  };
> >>>>>  DRM_ENUM_NAME_FN(drm_get_tv_mode_name, drm_tv_mode_enum_list)
> >>>>> =20
> >>>>> +/**
> >>>>> + * 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 v=
ery
> >>> well have an option after the tv_mode, something like
> >>> 720x480i,tv_mode=3DNTSC,rotate=3D180
> >>>
> >>> 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.
> >=20
> > 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
> >=20
>=20
> 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) =3D=3D 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"))) =3D=3D 0, while it obvio=
usly
isn't for us.

> Anyways:
>=20
> Reviewed-by: Noralf Tr=F8nnes <noralf@tronnes.org>

Thanks!
Maxime

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