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

[Jernej Škrabec]

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 at siol.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 at siol.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 at gmail.com>
> > > > > Signed-off-by: Jernej Skrabec <jernej.skrabec at siol.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 at 30) and CVBS. At least I 
didn't notice anything.

Best regards,
Jernej