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

[Maxime Ripard]

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

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