linux-kernel - Re: [PATCH 2/2] drm/sun4i: Handle DRM_MODE_FLAG_**SYNC

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Message-ID: <20180216155049.r2mc724nfluwrrbe@flea.lan>
Date:   Fri, 16 Feb 2018 16:50:49 +0100
From:   Maxime Ripard <maxime.ripard@...tlin.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 Thu, Feb 15, 2018 at 07:05:56PM +0100, Giulio Benetti wrote:
> > If so, and if remember the captures properly, the sampling would occur
> > right before the rise, and not really around the fall.
> > 
> > Would 2/3 be better here?
> 
> Yes, you're right, 2/3 phase is better:
> 
> 1/3 phase: https://pasteboard.co/H4VehON.png
> 2/3 phase: https://pasteboard.co/H4Veq8a.png
> 
> Take a look at the bit in middle(yellow) sampled by clock(blue).
> 
> Rising edge is almost in the middle of D0 bit.
> 
> > 
> > > According to scope captures above on both A20 and A33.
> > > Unfortunately I don't have other boards for the other SoCs to take captures.
> > > 
> > > What do you think?
> > 
> > I guess we can make that part applicable to all SoCs, we haven't seen
> > any significant differences on those part.
> 
> So let's keep:
> - As normal(rising edge) => IO_POL_REG "0x2 => 2/3 phase"
> - As inverted(falling edge) => IO_POL_REG "0x0 => normal phase"

I was actually thinking 1/3 for rising, 2/3 for falling.

Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
http://bootlin.com

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