Nouveau - Sep 2022 - [PATCH v2 10/41] drm/modes: Add a function to generate analog display modes

Hi Maxime,

W dniu 5.09.2022 o 15:37, Maxime Ripard pisze:
>>> +??? 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?
Well, increasing vbp only gives a centered image with the default 480i/576i
resolutions. However, only ever changing vbp will cause the image to be always
at the bottom of the screen when the active line count is decreased (e.g.
setting the resolution to 720x480 but for 50Hz "PAL" - like many game
consoles
did back in the day).

I believe that the perfect solution would:

- Use the canonical / standard-defined blanking line counts for the standard
? vertical resolutions (480/486/576)
- Increase vfp and vbp from there by the same number if a smaller number of
? active lines is specified, so that the resulting image is centered
- Likewise, decrease vfp and vbp by the same number if the active line number
? is larger and there is still leeway (this should allow for seamless handling
? of 480i vs. 486i for 60 Hz "NTSC")
- If even more active lines are specified, once the limit for vfp is hit, then
? decrease vbp only - the resulting image will definitely be off-center, but
? there's no other way

I hope this makes sense for you as well.

Best regards,
Mateusz Kwiatkowski

On Mon, Sep 05, 2022 at 06:44:42PM +0200, Mateusz Kwiatkowski
wrote:
> Hi Maxime,
> 
> W dniu 5.09.2022 o 15:37, Maxime Ripard pisze:
> >>> +??? 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?
> 
> Well, increasing vbp only gives a centered image with the default 480i/576i
> resolutions. However, only ever changing vbp will cause the image to be
always
> at the bottom of the screen when the active line count is decreased (e.g.
> setting the resolution to 720x480 but for 50Hz "PAL" - like many
game consoles
> did back in the day).
> 
> I believe that the perfect solution would:
> 
> - Use the canonical / standard-defined blanking line counts for the
standard
> ? vertical resolutions (480/486/576)
> - Increase vfp and vbp from there by the same number if a smaller number of
> ? active lines is specified, so that the resulting image is centered
> - Likewise, decrease vfp and vbp by the same number if the active line
number
> ? is larger and there is still leeway (this should allow for seamless
handling
> ? of 480i vs. 486i for 60 Hz "NTSC")
I'm not sure I understand how that's any different than the code you
initially commented on.

I would start by taking the entire blanking area, remove the sync
period. We only have the two porches now, and I'm starting from the
minimum, adding as many pixels in both (unless it's not an even number,
in which case the backporch will have the extra pixel).

Isn't it the same thing?
> - If even more active lines are specified, once the limit for vfp is hit,
then
> ? decrease vbp only - the resulting image will definitely be off-center,
but
> ? there's no other way
Unless you only want me to consider the front porch maximum?

Maxime
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