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Date:   Mon, 29 Aug 2022 15:29:53 +0200
From:   Maxime Ripard <maxime@cerno.tech>
To:     Mateusz Kwiatkowski <kfyatek@gmail.com>
Cc:     Geert Uytterhoeven <geert@linux-m68k.org>,
        Jernej Skrabec <jernej.skrabec@gmail.com>,
        Martin Blumenstingl <martin.blumenstingl@googlemail.com>,
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Subject: Re: [PATCH v1 04/35] drm/modes: Introduce 480i and 576i modes
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Hi Mateusz

On Wed, Aug 24, 2022 at 06:42:18PM +0200, Mateusz Kwiatkowski wrote:
> Hi Maxime,
>=20
> W dniu 18.08.2022 o 17:56, Geert Uytterhoeven pisze:
> > Hi Maxime,
> >
> > On Thu, Aug 18, 2022 at 5:46 PM Maxime Ripard <maxime@cerno.tech> wrote:
> >> On Thu, Aug 18, 2022 at 05:34:30PM +0200, Geert Uytterhoeven wrote:
> >>> On Thu, Aug 18, 2022 at 3:42 PM Maxime Ripard <maxime@cerno.tech> wro=
te:
> >>>> I started adding more sanity checks to my code, and I just realised I
> >>>> don't seem to be able to reach 720 pixels over a single line though.=
 If
> >>>> I understood it properly, and according to [1] the active part of a =
line
> >>>> is supposed to be 51.95us, and the blanking period taking 12.05us. [=
2]
> >>>> in the timing section has pretty much the same numbers, so it looks
> >>>> sane.
> >>>>
> >>>> At 13.5Mhz, a pixel is going to take roughly 74ns, and 51950 / 74 =
=3D 702
> >>>> pixels
> >>>>
> >>>> It seems we can go push it to 52350 ns, but that still gives us only=
 706
> >>>> pixels.
> >>>>
> >>>> Similarly, if I just choose to ignore that limit and just take the
> >>>> active time I need, 720 * 74 =3D 53280ns
> >>>>
> >>>> That leaves us 10720ns for the blanking period, and that's not enoug=
h to
> >>>> fit even the minimum of the front porch, hsync and back porch (1.55 +
> >>>> 4.5 + 5.5 =3D 11.55us).
> >>>>
> >>>> Are those constraints merely recommendations, or am I missing someth=
ing?
> >>>
> >>> You are missing that the parts near the borders of the full image are
> >>> part of the overscan range, and may or may not be visible, depending
> >>> on your actual display.
> >>> The full 768x576 image size from BT.656 is not visible on a typical P=
AL display,
> >>> and is more of an "absolute maximum rating", guaranteed to cover more
> >>> than analog PAL.
> >>
> >> So the overscan range is not part of the active area, unlike what HDMI
> >> is doing for example?
> >
> > Indeed. DVI-D and HDMI etc. are pure digital (let's ignore they are a
> > digitized variant of old analog VGA ;-), hence there is a one-to-one
> > match between pixels in the image and pixels on the screen (ignoring
> > scaling).=A0 But even when using an analog VGA input on a modern
> > digital display, you have controls to e.g. move the image.
> >
> >> Is there some minimal timings available somewhere to fit those absolute
> >> maximum ratings?
> >
> > I guess they can be found on the Internet...
>=20
> Here are some references that I personally found useful:
>=20
> - ITU-R BT.601 <https://www.itu.int/rec/R-REC-BT.601/en>
> =A0 This is *the* standard that pretty much every modern device that deal=
s with
> =A0 analog-style TV signal follows then converting to and from the digita=
l domain.
> =A0 For example in the figures on page 10 (12 in the PDF numbering) you c=
an see
> =A0 that the "time datum", i.e. start of horizontal sync pulse is canonic=
ally
> =A0 supposed to happen on sample 732 for 50 Hz or sample 736 for 59.94 Hz=
 modes.
>=20
> =A0 BT.601 assumes 13.5 MHz sample rate / pixel clock, but you can propor=
tionally
> =A0 scale those for other pixel clocks.
>=20
> - ITU-R BT.1700 <https://www.itu.int/rec/R-REC-BT.1700/en>
> =A0 This is *the* standard in force for actual analog composite video sig=
nals.
> =A0 The vertical sync specs are discrete, so they don't really change bet=
ween
> =A0 analog and digital domains. For horizontal sync, the values in those =
specs
> =A0 are given in microseconds/nanoseconds, but you can multiply those by =
the
> =A0 sampling rate for equivalent pixel counts.
>=20
> - Pembers' Ponderings
> =A0 <https://web.archive.org/web/20160423225838/http://www.pembers.freese=
rve.co.uk/>
> =A0 An old archived website with a ton of resources about analog TV.
> =A0 The "Line Standards" article will probably be most interesting to you.

Thanks so much for all those resources, it's been super helpful :)

> By the way, please note a couple of things:
>=20
> - The analog standards are very imprecise for modern digital norms, giving
> =A0 considerable leeway for just about every timing. The allowed leeways =
are
> =A0 usually equivalent to a couple of pixels at the standard 13.5 MHz sam=
pling
> =A0 rate - and those are meant for the transmitting end. Receivers are us=
ually
> =A0 much more forgiving to maximize compatibility.

Ok

> - The 720-pixel standard of BT.601 is considerably wider than the active =
width
> =A0 specified in the analog standards. AFAIK this is intentional, to ensu=
re that
> =A0 no part of the actual image is missed during digitization, and to kee=
p the
> =A0 number a nice multiply of 16. The picture width given in the analog s=
tandards
> =A0 is equivalent to somewhere between 702 and 714 pixels (at 13.5 MHz cl=
ock),
> =A0 depending on the specific standard. And that includes overscan.

Ok. I think it still makes sense to allow it, if only we were using it so f=
ar :)

I've done a first implementation in the v2 I just sent that seems to
work ok, please let me know if I did anything stupid :)

In particular, I chose, if we were between 702 and 720 pixels to disable
all duration checks, and take the missing time from the front and back
porch, in equal proportions.

> - Same goes for the vertical active area. Original analog standards varied
> =A0 wildly from country to country, before finally settling on 575 lines =
for the
> =A0 50 Hz standard and 485 lines for the 59.94 Hz standard. Or 576/486, d=
epending
> =A0 on how you count. The topmost line of those 576/486 starts at half th=
e screen,
> =A0 and the bottommost line ends at half the screen - so they are often c=
ombined
> =A0 when counting and given as 575/485. The digital 576i50 standard inclu=
des
> =A0 those half-lines. In the 59.94 Hz regions, 480 active digial lines en=
ded up
> =A0 the norm, because 486 does not have nice dividers, and also some of t=
he
> =A0 outermost lines which were always overscanned anyway, ended up used f=
or things
> =A0 like closed captioning over the years.

Ok

> - Speaking of closed captioning... a lot of different stuff were put in t=
he
> =A0 blanking interval over the years. Like teletext in Europe. There are =
projects
> =A0 like VBIT2 <https://github.com/peterkvt80/vbit2> which intentionally
> =A0 reconfigure the Raspberry Pi composite output to include the blanking=
 interval
> =A0 in the framebuffer so that teletext can be output by drawing on the e=
dge of
> =A0 the "screen" (from the computer point of view).

I'm not sure how we would support this in KMS to be honest. Asking for a
wider mode and the userspace putting whatever it wants in the margins
seems like a good choice.

> - A lot of equipment outside the broadcast industry willingly violated th=
ose
> =A0 standards, and there are real world use cases for that. Film studios =
used very
> =A0 slightly modified TVs to make them sync with 24fps cameras - in that =
variant,
> =A0 "NTSC" could have e.g. 655 lines so that the TV would refresh at 48 H=
z with
> =A0 the same line frequency. Home computers and video game consoles output
> =A0 progressive 262/312-line modes instead of interlaced 525/625 lines. A=
nd often
> =A0 changed the line frequency slightly as well, for various reasons. Tho=
se
> =A0 progressive modes are still favored by retro gaming and emulation ent=
husiasts,
> =A0 because they incur a specific look on CRT displays. Even playing back=
 video
> =A0 from a tape (especially home-grade, like VHS) could cause timings to =
go wildly
> =A0 out of spec, because of mechanical imprecisions.

Ok

> - There were multitude of standards predating the ubiquitous 525/60 and 6=
25/50
> =A0 modes. The British 405-line and French 819-line standards are the most
> =A0 notorious, having lasted well into the 1980s, but there were also a l=
ot of
> =A0 wildly varying pre-WW2 television systems. And there are enthusiasts =
dedicated
> =A0 to preserving those.
>=20
> My point is that the norms for analog TV are rather loose, and I think we
> shouldn't limit the drivers to only accepting the "proper" modes as defin=
ed in
> the spec. Those should of course be the default, but if non-standard mode=
lines
> can be generated - there are legitimate use cases why people might want t=
hose.

Yep, that part has been dropped. I'm still wondering if we'd need to
still have a bunch of restrictions (like a total number of lines of 625
with NTSC would be obviously invalid), but that can always be added
later on if such a need comes up

Maxime

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