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Date:   Thu, 30 Apr 2020 15:54:38 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Xin Ji <xji@analogixsemi.com>
Cc:     Daniel Vetter <daniel@ffwll.ch>,
        Neil Armstrong <narmstrong@baylibre.com>,
        Jonas Karlman <jonas@kwiboo.se>,
        Jernej Skrabec <jernej.skrabec@siol.net>,
        David Airlie <airlied@linux.ie>,
        Dan Carpenter <dan.carpenter@oracle.com>,
        Nicolas Boichat <drinkcat@chromium.org>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        Pi-Hsun Shih <pihsun@chromium.org>,
        Sheng Pan <span@analogixsemi.com>
Subject: Re: [PATCH v7 2/2] drm/bridge: anx7625: Add anx7625 MIPI DSI/DPI to
 DP bridge driver
Message-ID: <20200430135438.GD10381@phenom.ffwll.local>
Mail-Followup-To: Xin Ji <xji@analogixsemi.com>,
        Neil Armstrong <narmstrong@baylibre.com>,
        Jonas Karlman <jonas@kwiboo.se>,
        Jernej Skrabec <jernej.skrabec@siol.net>,
        David Airlie <airlied@linux.ie>,
        Dan Carpenter <dan.carpenter@oracle.com>,
        Nicolas Boichat <drinkcat@chromium.org>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        Pi-Hsun Shih <pihsun@chromium.org>,
        Sheng Pan <span@analogixsemi.com>
References: <cover.1582529411.git.xji@analogixsemi.com>
 <a81adcf2e79d440edcb7b3989f31efcb80a6e9ff.1582529411.git.xji@analogixsemi.com>
 <CANMq1KBfB6tXFqYGvr=8fV_bpCV5GbVHeEbRs+fuaZba65-OPw@mail.gmail.com>
 <20200424065124.GA31922@xin-VirtualBox>
 <CANMq1KBJ6f74aNAr8BwC3wz8MEeJzwXOQE44gv6C=DNzYmUWCQ@mail.gmail.com>
 <20200428100508.GD3456981@phenom.ffwll.local>
 <20200430033614.GA6645@xin-VirtualBox>
 <20200430133731.GA10381@phenom.ffwll.local>
 <20200430133839.GB10381@phenom.ffwll.local>
 <20200430134746.GA2188@xin-VirtualBox>
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On Thu, Apr 30, 2020 at 09:47:46PM +0800, Xin Ji wrote:
> Hi Daniel,
> 
> On Thu, Apr 30, 2020 at 03:38:39PM +0200, Daniel Vetter wrote:
> > On Thu, Apr 30, 2020 at 03:37:31PM +0200, Daniel Vetter wrote:
> > > On Thu, Apr 30, 2020 at 11:36:14AM +0800, Xin Ji wrote:
> > > > On Tue, Apr 28, 2020 at 12:05:08PM +0200, Daniel Vetter wrote:
> > > > > On Fri, Apr 24, 2020 at 08:12:04PM +0800, Nicolas Boichat wrote:
> > > > > > On Fri, Apr 24, 2020 at 2:51 PM Xin Ji <xji@analogixsemi.com> wrote:
> > > > > > >
> > > > > > > On Thu, Apr 23, 2020 at 07:55:15PM +0800, Nicolas Boichat wrote:
> > > > > > > > Hi,
> > > > > > > >
> > > > > > > > Just commenting on the mode_fixup function that was added in v7.
> > > > > > > >
> > > > > > > [snip]
> > > > > > > > > +       /*
> > > > > > > > > +        * once illegal timing detected, use default HFP, HSYNC, HBP
> > > > > > > > > +        */
> > > > > > > > > +       if (hblanking < HBLANKING_MIN || (hfp < HP_MIN && hbp < HP_MIN)) {
> > > > > > > >
> > > > > > > > should this be adj_hblanking/adj_hfp/adj_hbp?
> > > > > > > NO, need check original HFP and HBP, if they are not legal, driver need
> > > > > > > set default value to adj_hsync, adj_hfp, adj_hbp.
> > > > > > > >
> > > > > > > > > +               adj_hsync = SYNC_LEN_DEF;
> > > > > > > > > +               adj_hfp = HFP_HBP_DEF;
> > > > > > > > > +               adj_hbp = HFP_HBP_DEF;
> > > > > > > > > +               vref = adj->clock * 1000 / (adj->htotal * adj->vtotal);
> > > > > > > > > +               if (hblanking < HBLANKING_MIN) {
> > > > > > > > > +                       delta_adj = HBLANKING_MIN - hblanking;
> > > > > > > > > +                       adj_clock = vref * delta_adj * adj->vtotal;
> > > > > > > > > +                       adj->clock += DIV_ROUND_UP(adj_clock, 1000);
> > > > > > > > > +               } else {
> > > > > > > > > +                       delta_adj = hblanking - HBLANKING_MIN;
> > > > > > > > > +                       adj_clock = vref * delta_adj * adj->vtotal;
> > > > > > > > > +                       adj->clock -= DIV_ROUND_UP(adj_clock, 1000);
> > > > > > > > > +               }
> > > > > > > > > +
> > > > > > > > > +               DRM_WARN("illegal hblanking timing, use default.\n");
> > > > > > > > > +               DRM_WARN("hfp(%d),hbp(%d),hsync(%d).\n", hfp, hbp, hsync);
> > > > > > > >
> > > > > > > > How likely is it that this mode is going to work? Can you just return
> > > > > > > > false here to reject the mode?
> > > > > > > We want to set the default minimal Hblancking value, then it may display,
> > > > > > > otherwise. If we just return false, there is no display for sure.
> > > > > > 
> > > > > > Right, understand your argument. I'm pondering if it's not just better
> > > > > > to reject the mode rather than trying a timing that is definitely
> > > > > > quite different from what the monitor was asking for. No super strong
> > > > > > opinion, I'll let other people on the list weigh in.
> > > > > 
> > > > > Yeah mode_fixup is supposed to be used to adjust the mode in intermediate
> > > > > stages (e.g. if you go from progressive to interlaced only at the end of
> > > > > your pipeline or something like that). It's not meant for adjusting the
> > > > > mode yout actually put out through a hdmi or dp connector. For fixed
> > > > > panels adjusting modes to fit the panel is also fairly common, but not for
> > > > > external outputs.
> > > > > 
> > > > > Since this is a DP bridge I'd say no adjusting, just reject what doesn't
> > > > > fit.
> > > > We have found some panel which HBP less than 8, if we reject to adjust
> > > > video timing, then there is no display. The customer does not accept it,
> > > > they push us to fix it, the only resolve way is to adjust timing.
> > > 
> > > Are we talking about external DP screen here, or some built-in panel? For
> > > the later case we do a lot of mode adjusting in many drivers ...
> > > 
> > > I haven't checked, by if our connector type is eDP then this should be all
> > > fine.
> > 
> > Ok I read the patch now, you seem to support both. Would it work if we
> > make this adjustement conditional on it being an internal panel only? I
> > think that would be perfect.
> > -Daniel
> > -- 
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> Based on comments of V8, only keeped eDP built-in panel in V9 version,
> removed external DP screen support.

Ah even better. Then the above adjusting has my:

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Maybe add a comment to the code summarizing the discussion. Definitely
needs to be covered in the commit message.

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch