linux-kernel - Re: [PATCH v2 10/41] drm/modes: Add a function to generate analog display modes

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20220905133755.gcmmntg3wnecyqjq@houat>
Date:   Mon, 5 Sep 2022 15:37:55 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Mateusz Kwiatkowski <kfyatek@...il.com>
Cc:     Ben Skeggs <bskeggs@...hat.com>, David Airlie <airlied@...ux.ie>,
        Chen-Yu Tsai <wens@...e.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Lyude Paul <lyude@...hat.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Samuel Holland <samuel@...lland.org>,
        Karol Herbst <kherbst@...hat.com>,
        Noralf Trønnes <noralf@...nnes.org>,
        Emma Anholt <emma@...olt.net>, Daniel Vetter <daniel@...ll.ch>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Hans de Goede <hdegoede@...hat.com>,
        linux-arm-kernel@...ts.infradead.org,
        Phil Elwell <phil@...pberrypi.com>,
        intel-gfx@...ts.freedesktop.org,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        dri-devel@...ts.freedesktop.org, Dom Cobley <dom@...pberrypi.com>,
        linux-kernel@...r.kernel.org, nouveau@...ts.freedesktop.org,
        linux-sunxi@...ts.linux.dev,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v2 10/41] drm/modes: Add a function to generate analog
 display modes

Hi,

On Wed, Aug 31, 2022 at 03:44:52AM +0200, Mateusz Kwiatkowski wrote:
> > +#define NTSC_HFP_DURATION_TYP_NS    1500
> > +#define NTSC_HFP_DURATION_MIN_NS    1270
> > +#define NTSC_HFP_DURATION_MAX_NS    2220
> 
> You've defined those min/typ/max ranges, but you're not using the "typ" field
> for anything other than hslen.

Yeah... I've left most of them because it was so hard to find most of
them, it's useful at least for documentation purposes. And it's a define
so there's pretty much no downside to it as far as the final binary is
involved.

> The actual "typical" value is thus always the midpoint, which isn't
> necessarily the best choice.
> 
> In particular, for the standard 720px wide modes at 13.5 MHz, hsync_start
> ends up being 735 for 480i and 734 for 576i, instead of 736 and 732 requested
> by BT.601. That's all obviously within tolerances, but the image ends up
> noticeably off-center (at least on modern TVs), especially in the 576i case.

I'll try to fix that up.

> > +    htotal = params->line_duration_ns * pixel_clock_hz / NSEC_PER_SEC;
> 
> You're multiplying an unsigned int and an unsigned long - both types are only
> required to be 32 bit, so this is likely to overflow. You need to use a cast to
> unsigned long long, and then call do_div() for 64-bit division.
> 
> This actually overflowed on me on my Pi running ARM32 kernel, resulting in
> negative horizontal porch lengths, and drm_helper_probe_add_cmdline_mode()
> taking over the mode generation (badly), and a horrible mess on screen.

Indeed, that's bad.

> > +    vfp = vfp_min + (porches_rem / 2);
> > +    vbp = porches - vfp;
> 
> Relative position of the vertical sync within the VBI effectively moves the
> image up and down. Adding that (porches_rem / 2) moves the image up off center
> by that many pixels. I'd keep the VFP always at minimum to keep the image
> centered.

And you would increase the back porch only then?

Maxime

Download attachment "signature.asc" of type "application/pgp-signature" (229 bytes)