Re: [PATCH] drm/vc4: vec: Add support for PAL-60

[maxime]

On Tue, Oct 18, 2022 at 10:57:04PM +0200, Mateusz Kwiatkowski wrote:
> Hi Maxime,
> 
> W dniu 18.10.2022 o 10:31, Maxime Ripard pisze:
> > Hi,
> >
> > On Sun, Oct 16, 2022 at 09:46:49PM +0200, Mateusz Kwiatkowski wrote:
> >> @@ -308,14 +324,15 @@ static const struct vc4_vec_tv_mode vc4_vec_tv_modes[] = {
> >>  };
> >>  
> >>  static inline const struct vc4_vec_tv_mode *
> >> -vc4_vec_tv_mode_lookup(unsigned int mode)
> >> +vc4_vec_tv_mode_lookup(unsigned int mode, u16 htotal)
> >>  {
> >>  	unsigned int i;
> >>  
> >>  	for (i = 0; i < ARRAY_SIZE(vc4_vec_tv_modes); i++) {
> >>  		const struct vc4_vec_tv_mode *tv_mode = &vc4_vec_tv_modes[i];
> >>  
> >> -		if (tv_mode->mode == mode)
> >> +		if (tv_mode->mode == mode &&
> >> +		    tv_mode->expected_htotal == htotal)
> >>  			return tv_mode;
> >
> > Is there any reason we're not using the refresh rate to filter this? It
> > seems more natural to me.
> 
> Let me give you an example first.
> 
> There are actually two ways to configure PAL-60-ish mode on VC4/VEC:
> 
> a) Modeline 13.5 720 734 798 858 480 487 493 525 Interlace, standard registers
>    set to VEC_CONFIG0_PAL_M_STD, custom frequency enabled and set to 0x2a098acb;
>    Setting the standard registers to "PAL-M" puts the VEC in true 59.94 Hz mode,
>    so the video timings are identical as for NTSC (or PAL-M), and the custom
>    frequency makes the color subcarrier compatible with regular PAL receivers.
>    This is the "true" PAL-60, thanks to the true System M timings.

That's the one I would expect, and I assume we could just do that by
selecting the 480i mode + PAL TV Mode property, right?

> a) Modeline 13.5 720 740 804 864 480 486 492 525 Interlace, standards registers
>    set to VEC_CONFIG0_PAL with standard frequency; This is a "fake" PAL-60 mode,
>    the refresh rate is actually ~59.524 Hz. Most "NTSC" sets will be able to
>    sync with this mode no problem, but the VEC is actually operating in its
>    50 Hz mode - it's just the "premature" vertical sync signal causes it to
>    output something that is similar to the 525-line system, however strictly
>    speaking non-standard due to lower horizontal sync frequency.

But it's not really clear to me why we should support both.

> This comes down to the fact that:
> 
> - When VEC's standard registers are set to VEC_CONFIG0_NTSC_STD or
>   VEC_CONFIG0_PAL_M_STD, it operates in the "CCIR System M" mode, expects htotal
>   to be exactly 858 pixels (and it will generate horizontal sync pulse every 858
>   pixels on its own regardless of what comes out of the PV - so there will be
>   garbage on screen if you set it to anything else), and vtotal to be 525 lines.
>   It will not accept vtotal that's any higher (it will generate its own vertical
>   sync as demanded by System M if not triggered by the PV), but it can be lower
>   - resulting in modes that are non-standard, but otherwise valid.
> 
> - Likewise, when the registers are set to VEC_CONFIG0_PAL_BDGHI_STD,
>   VEC_CONFIG0_PAL_N_STD or VEC_CONFIG0_SECAM_STD (SECAM is a bit special, but
>   that's irrelevant here), it operates in the "CCIR System B/D/G/H/I/N" mode,
>   and likewise, expects htotal to be exactly 864 pixels (garbage on screen
>   otherwise), vtotal limit is 625 lines, etc.
> 
> Checking for the refresh rate would only work for standard-compliant modes and
> have the potential of completely breaking on any custom modes. Conversely,
> checking for htotal aligns perfectly with the limitations of the hardware, and
> allows the user to set any modeline that the hardware is able to output with
> any level of sanity.

OK

Thanks!
Maxime

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