linux-kernel - Re: [linux-sunxi] [PATCH 3/3] Revert "drm/sun4i: dsi: Rework a bit the hblk calculation"

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Date:   Sun, 06 Oct 2019 23:12:43 +0800
From:   Icenowy Zheng <icenowy@...c.io>
To:     Jagan Teki <jagan@...rulasolutions.com>
Cc:     Maxime Ripard <mripard@...nel.org>, Chen-Yu Tsai <wens@...e.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>
Subject: Re: [linux-sunxi] [PATCH 3/3] Revert "drm/sun4i: dsi: Rework a bit
 the hblk calculation"

在 2019-10-06日的 22:44 +0800，Icenowy Zheng写道：
> 在 2019-10-03四的 09:53 +0530，Jagan Teki写道：
> > Hi Wens,
> > 
> > On Tue, Oct 1, 2019 at 1:34 PM Icenowy Zheng <icenowy@...c.io>
> > wrote:
> > > This reverts commit 62e7511a4f4dcf07f753893d3424decd9466c98b.
> > > 
> > > This commit, although claimed as a refactor, in fact changed the
> > > formula.
> > > 
> > > By expanding the original formula, we can find that the const 10
> > > is
> > > not
> > > substracted, instead it's added to the value (because 10 is
> > > negative
> > > when calculating hsa, and hsa itself is negative when calculating
> > > hblk).
> > > This breaks the similar pattern to other formulas, so restoring
> > > the
> > > original formula is more proper.
> > > 
> > > Signed-off-by: Icenowy Zheng <icenowy@...c.io>
> > > ---
> > >  drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c | 9 ++-------
> > >  1 file changed, 2 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > > b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > > index 2d3e822a7739..cb5fd19c0d0d 100644
> > > --- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > > +++ b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > > @@ -577,14 +577,9 @@ static void sun6i_dsi_setup_timings(struct
> > > sun6i_dsi *dsi,
> > >                           (mode->hsync_start - mode->hdisplay) *
> > > Bpp - HFP_PACKET_OVERHEAD);
> > > 
> > >                 /*
> > > -                * The blanking is set using a sync event (4
> > > bytes)
> > > -                * and a blanking packet (4 bytes + payload + 2
> > > -                * bytes). Its minimal size is therefore 10
> > > bytes.
> > > +                * hblk seems to be the line + porches length.
> > >                  */
> > > -#define HBLK_PACKET_OVERHEAD   10
> > > -               hblk = max((unsigned int)HBLK_PACKET_OVERHEAD,
> > > -                          (mode->htotal - (mode->hsync_end -
> > > mode-
> > > > hsync_start)) * Bpp -
> > > -                          HBLK_PACKET_OVERHEAD);
> > > +               hblk = mode->htotal * Bpp - hsa;
> > 
> > The original formula is correct according to BSP [1] and work with
> > my
> > panels which I have tested before. May be the horizontal timings on
> > panels you have leads to negative value.
> 
> Do you tested the same timing with BSP kernel?
> 
> It's quite difficult to get a negative value here, because the value
> is
> quite big (includes mode->hdisplay * Bpp).

By re-checking with the BSP source code, I found that the constant in
the HFP formula is indeed wrong -- it should be 16, not 6.

> 
> Strangely, only change the formula here back makes the timing
> translated from FEX file works (tested on PineTab and PinePhone
> production ver). The translation rule is from [1].
> 
> So I still insist on the patch because it's needed by experiment.
> 
> [1] http://linux-sunxi.org/LCD
> 
> > [1] 
> > https://github.com/ayufan-pine64/linux-pine64/blob/my-hacks-1.2-with-drm/drivers/video/sunxi/disp2/disp/de/lowlevel_sun50iw1/de_dsi.c#L919