linux-kernel - Re: [PATCH 2/5] drm/sun4i: tcon: set sync polarity for tcon1 channel

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Date:   Fri, 5 Feb 2021 17:01:30 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     Jernej Skrabec <jernej.skrabec@...l.net>,
        Mike Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        linux-clk <linux-clk@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>,
        Andre Heider <a.heider@...il.com>
Subject: Re: [PATCH 2/5] drm/sun4i: tcon: set sync polarity for tcon1 channel

On Fri, Feb 05, 2021 at 11:21:22AM +0800, Chen-Yu Tsai wrote:
> On Fri, Feb 5, 2021 at 2:48 AM Jernej Skrabec <jernej.skrabec@...l.net> wrote:
> >
> > Channel 1 has polarity bits for vsync and hsync signals but driver never
> > sets them. It turns out that with pre-HDMI2 controllers seemingly there
> > is no issue if polarity is not set. However, with HDMI2 controllers
> > (H6) there often comes to de-synchronization due to phase shift. This
> > causes flickering screen. It's safe to assume that similar issues might
> > happen also with pre-HDMI2 controllers.
> >
> > Solve issue with setting vsync and hsync polarity. Note that display
> > stacks with tcon top have polarity bits actually in tcon0 polarity
> > register.
> >
> > Fixes: 9026e0d122ac ("drm: Add Allwinner A10 Display Engine support")
> > Tested-by: Andre Heider <a.heider@...il.com>
> > Signed-off-by: Jernej Skrabec <jernej.skrabec@...l.net>
> > ---
> >  drivers/gpu/drm/sun4i/sun4i_tcon.c | 24 ++++++++++++++++++++++++
> >  drivers/gpu/drm/sun4i/sun4i_tcon.h |  5 +++++
> >  2 files changed, 29 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/sun4i/sun4i_tcon.c b/drivers/gpu/drm/sun4i/sun4i_tcon.c
> > index 6b9af4c08cd6..0d132dae58c0 100644
> > --- a/drivers/gpu/drm/sun4i/sun4i_tcon.c
> > +++ b/drivers/gpu/drm/sun4i/sun4i_tcon.c
> > @@ -672,6 +672,29 @@ static void sun4i_tcon1_mode_set(struct sun4i_tcon *tcon,
> >                      SUN4I_TCON1_BASIC5_V_SYNC(vsync) |
> >                      SUN4I_TCON1_BASIC5_H_SYNC(hsync));
> >
> > +       /* Setup the polarity of sync signals */
> > +       if (tcon->quirks->polarity_in_ch0) {
> > +               val = 0;
> > +
> > +               if (mode->flags & DRM_MODE_FLAG_PHSYNC)
> > +                       val |= SUN4I_TCON0_IO_POL_HSYNC_POSITIVE;
> > +
> > +               if (mode->flags & DRM_MODE_FLAG_PVSYNC)
> > +                       val |= SUN4I_TCON0_IO_POL_VSYNC_POSITIVE;
> > +
> > +               regmap_write(tcon->regs, SUN4I_TCON0_IO_POL_REG, val);
> > +       } else {
> > +               val = SUN4I_TCON1_IO_POL_UNKNOWN;
> 
> I think a comment for the origin of this is warranted.

If it's anything like TCON0, it's the pixel clock polarity

Maxime