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

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Message-ID: <20210209103158.rnzvlvo44nnzrmk4@gilmour>
Date:   Tue, 9 Feb 2021 11:31:58 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Jernej Škrabec <jernej.skrabec@...l.net>
Cc:     Chen-Yu Tsai <wens@...e.org>,
        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 07:47:17PM +0100, Jernej Škrabec wrote:
> Dne petek, 05. februar 2021 ob 17:28:23 CET je Chen-Yu Tsai napisal(a):
> > On Sat, Feb 6, 2021 at 12:21 AM Jernej Škrabec <jernej.skrabec@...l.net> 
> wrote:
> > >
> > > Dne petek, 05. februar 2021 ob 17:01:30 CET je Maxime Ripard napisal(a):
> > > > 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
> > >
> > > Hard to say, DW HDMI controller has "data enable" polarity along hsync and
> > > vsync. It could be either or none of those.
> > >
> > > What should I write in comment? BSP drivers and documentation use only 
> generic
> > > names like io2_inv.
> > 
> > Just say that we don't know exactly what it is, but it is required for 
> things
> > to work properly? Would be interesting to know what happens if you don't set
> > this bit, but do set VSYNC/HSYNC polarity properly.
> 
> Nothing seems to happen - tested on H3 with HDMI (4k@30) and CVBS. At least I 
> didn't notice anything.

That's pretty normal, an inverted pixel clock would at worst give you
some weird artifacts and / or pixels being of the wrong color. Data
enable on the other hand would very likely stall the HDMI controller
since you would have only the blanking periods that would be considered
valid.

Maxime

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