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Date:   Sat, 29 Sep 2018 15:53:02 +0200
From:   Maxime Ripard <maxime.ripard@bootlin.com>
To:     Jagan Teki <jagan@amarulasolutions.com>
Cc:     Chen-Yu Tsai <wens@csie.org>, Icenowy Zheng <icenowy@aosc.io>,
        Jernej Skrabec <jernej.skrabec@siol.net>,
        Vasily Khoruzhick <anarsoul@gmail.com>,
        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        David Airlie <airlied@linux.ie>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Michael Turquette <mturquette@baylibre.com>,
        Stephen Boyd <sboyd@kernel.org>,
        linux-clk <linux-clk@vger.kernel.org>,
        Michael Trimarchi <michael@amarulasolutions.com>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        devicetree <devicetree@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-sunxi <linux-sunxi@googlegroups.com>
Subject: Re: [PATCH 06/12] drm/sun4i: sun6i_mipi_dsi: Fix VBP size calculation
Message-ID: <20180929135302.ymdzapz5npvapti2@flea>
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On Thu, Sep 27, 2018 at 09:50:34PM +0530, Jagan Teki wrote:
> On Thu, Sep 27, 2018 at 8:51 PM Maxime Ripard <maxime.ripard@bootlin.com> wrote:
> >
> > On Thu, Sep 27, 2018 at 05:18:44PM +0530, Jagan Teki wrote:
> > > According to horizontal and vertical timings are defined
> > > per the diagram from include/drm/drm_modes.h
> > >
> > > Back porch = [hv]total - [hv]sync_end
> > >
> > > So, update SUN6I_DSI_BASIC_SIZE0_VBP calculation as
> > > mode->vtotal - mode->vsync_end
> > >
> > > Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
> > > ---
> > >  drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > > index 1c7e42015645..599284971ab6 100644
> > > --- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > > +++ b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > > @@ -526,8 +526,8 @@ static void sun6i_dsi_setup_timings(struct sun6i_dsi *dsi,
> > >       regmap_write(dsi->regs, SUN6I_DSI_BASIC_SIZE0_REG,
> > >                    SUN6I_DSI_BASIC_SIZE0_VSA(mode->vsync_end -
> > >                                              mode->vsync_start) |
> > > -                  SUN6I_DSI_BASIC_SIZE0_VBP(mode->vsync_start -
> > > -                                            mode->vdisplay));
> > > +                  SUN6I_DSI_BASIC_SIZE0_VBP(mode->vtotal -
> > > +                                            mode->vsync_end));
> >
> > Is it purely theoretical, or did you find some source that back that?
> 
> VSA is done as per that, sync_end - sync start would give sync time.

That's a different register though.

> VBP also done in BPI-M64-bsp[1] which results back porch existing code
> results fron porch.
> 
> [1] https://github.com/BPI-SINOVOIP/BPI-M64-bsp/blob/master/linux-sunxi/drivers/video/sunxi/disp2/disp/de/lowlevel_sun50iw1/de_dsi.c#L955

That code does back porch - sync length. Such a calculation doesn't
make much sense as is, but you're saying that it results in the front
porch. Again, what makes you say that?

Maxime

-- 
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com