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From:   Jani Nikula <jani.nikula@linux.intel.com>
To:     Maxime Ripard <maxime@cerno.tech>
Cc:     Geert Uytterhoeven <geert@linux-m68k.org>,
        Ben Skeggs <bskeggs@redhat.com>,
        David Airlie <airlied@linux.ie>, Chen-Yu Tsai <wens@csie.org>,
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        Philipp Zabel <p.zabel@pengutronix.de>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
        Jernej Skrabec <jernej.skrabec@gmail.com>,
        Samuel Holland <samuel@sholland.org>,
        Karol Herbst <kherbst@redhat.com>,
        Noralf =?utf-8?Q?Tr=C3=B8nnes?= <noralf@tronnes.org>,
        Emma Anholt <emma@anholt.net>, Daniel Vetter <daniel@ffwll.ch>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Hans de Goede <hdegoede@redhat.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Phil Elwell <phil@raspberrypi.com>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        Dave Stevenson <dave.stevenson@raspberrypi.com>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Dom Cobley <dom@raspberrypi.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Mateusz Kwiatkowski <kfyatek+publicgit@gmail.com>
Subject: Re: [PATCH v2 14/41] drm/modes: Move named modes parsing to a
 separate function
In-Reply-To: <20220830120330.6f5f22d35gu7cbr3@houat>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
References: <20220728-rpi-analog-tv-properties-v2-0-459522d653a7@cerno.tech>
 <20220728-rpi-analog-tv-properties-v2-14-459522d653a7@cerno.tech>
 <CAMuHMdV9wVgHFfwHoqtBoYzJDnjDmKTfaZkAKvTVKh1Y-2x1pA@mail.gmail.com>
 <87czcidnb8.fsf@intel.com> <20220830120330.6f5f22d35gu7cbr3@houat>
Date:   Tue, 30 Aug 2022 16:36:23 +0300
Message-ID: <875yi9etuw.fsf@intel.com>
MIME-Version: 1.0
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On Tue, 30 Aug 2022, Maxime Ripard <maxime@cerno.tech> wrote:
> Hi,
>
> On Tue, Aug 30, 2022 at 01:43:07PM +0300, Jani Nikula wrote:
>> On Tue, 30 Aug 2022, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>> > On Mon, Aug 29, 2022 at 3:13 PM Maxime Ripard <maxime@cerno.tech> wrote:
>> >> +#define STR_STRICT_EQ(str, len, cmp) \
>> >> +       ((strlen(cmp) == len) && !strncmp(str, cmp, len))
>> >
>> > This is not part of the move, but newly added.
>> 
>> The same construct is also duplicated elsewhere in the series, and I
>> kept being confused by it.
>
> I'm not sure what is confusing, but I can add a comment if needed.

STR_STRICT_EQ() is what's confusing. I have to look at the
implementation to understand what it means. What does "strict" string
equality mean?

>
>> The above is precisely the same as:
>> 
>> 	str_has_prefix(str, cmp) == len
>
> Here, it's used to make sure we don't have a named mode starting with
> either e, d, or D.
>
> If I understood str_has_prefix() right, str_has_prefix("DUMB-MODE", "D")
> == strlen("DUMB-MODE") would return true, while it's actually what we
> want to avoid.

That's not true, str_has_prefix("DUMB-MODE", "D") == strlen("D") is.

> It's also used indeed in drm_get_tv_mode_from_name(), where we try to
> match a list of names with one passed as argument.
>
> With drm_get_tv_mode_from_name("NSTC", strlen("NTSC")), we would end up
> calling str_has_prefix("NTSC-J", "NTSC") == strlen("NTSC-J") which would
> work. However, we end up calling prefix not a prefix, but an entire
> string we want to match against, which is very confusing to me too.

If I get this right, you have a string and you want to check if that has
a certain prefix. Additionally, you want to check the prefix is a
certain length.

Sure, that the prefix is a certain length is more of a property of the
string, which is NUL terminated later than at length, but that's doesn't
really matter.

That condition is simply str_has_prefix(string, prefix) == length.

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Graphics Center