Date:   Thu, 1 Feb 2018 11:14:11 +0100
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Giulio Benetti <giulio.benetti@...ronovasrl.com>
Cc:     airlied@...ux.ie, wens@...e.org, dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] drm/sun4i: Handle DRM_MODE_FLAG_**SYNC_POSITIVE
 correctly

On Sat, Jan 27, 2018 at 11:07:09PM +0100, Giulio Benetti wrote:
> > > > > I don't really know what the polarity of D0 would be just by
> > > > > judging at that capture, but we would have noticed if the colors
> > > > > were inverted for quite some time now.
> > > > 
> > > > D0-D23 are correct.
> > > > 
> > > > With that capture, I mean to show you instead dclk is inverted, as
> > > > dclk samples D0 on falling edge.
> > > 
> > > Ah right, DCLK being the first channel?
> > 
> > Yes, sorry I didn't place a label on channels
> > 
> > > 
> > > > So 0 is NEGEDGE and 1 is POSEDGE(1/3 of clock phase).
> > > > 1/3 clock phase seems enough to me to be considered POSEDGE,
> > > > 2/3 instead risks to go too much to the right of D0(even if it
> > > > could work).
> > > 
> > > Do you have captures with both settings?
> > 
> > Not now, but asap I'm going to take.
> 
> Here we are:
> 1/3 phase: https://pasteboard.co/H4VehON.png
> 2/3 phase: https://pasteboard.co/H4Veq8a.png
> 
> Yellow: D0
> Blue: DCLK
> 
> As you can see:
> 1/3 phase has DCLK rising edge almost in the middle of D0
> 2/3 phase has DCLK rising edge that comes too late
> 
> I would go for "1/3 phase" for Rising edge and "normal phase" for
> Falling edge.
> 
> What do you think?

It seems fair. This need a whole lot of comments though :)

> > > That it's going to be a nightmare... We've advertised since the very
> > > beginning something, and we're about to break it. I'm not sure we want
> > > to do that.
> > 
> > I can take care about that.
> > But I also think that a lot of displays work because they use only
> > DE-mode, almost ignoring HSync and VSync signals(HV-mode).
> > 
> > In any case I have to produce these patches because of my company's
> > board based on A20 and A33, and modify defconfig according to it.
> > The only technical nightmare I see is to produce a commit for every
> > defconfig to be modified and copy-paste che commit-log substituing board
> > name(1-2 days of work).
> > Problem is testing, but we're speaking about something that probably was
> > badly working, but you couldn't see it on display.
> > So I think this is only an improvement at the end.
> > 
> > I'm sorry I've taken bad news.
> > Drink 1 glass of Spritz to go over! :)
> 
> IMHO I think that we have only to take care about displays that don't have
> DE signal.
>
> If DE signal exists, then displays are driven through DE only for back and
> front porch as I know, and on most displays I've used, Hsync and VSync are
> ignored.
> DE is used not only for Data Enable, but also for sync the very beginning of
> frame, the rest of syncing is done by pause between every line sent.
> This is should be why nobody noticed it before,
> I think almost every display is used in DE mode only.
> So, if we fix bug for HSync and VSync, risk should be very low.
> Indeed, everybody or almost, use sync:3 because display ignore those 2
> signals (HSync and VSYnc) in favour of DE.
> And I don't know how many people checked with oscilloscope signals after
> getting display working in a few.

I know I did, but I apparently didn't pay attention to that and was
more focused on getting the timings right :)

But clearly this is a separate discussion that needs to be held on the
U-Boot ML.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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