linux-kernel - Re: [PATCH v7 2/2] drm/bridge: anx7625: Add anx7625 MIPI DSI/DPI to DP bridge driver

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Message-ID: <20200428100508.GD3456981@phenom.ffwll.local>
Date:   Tue, 28 Apr 2020 12:05:08 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Nicolas Boichat <drinkcat@...omium.org>
Cc:     Xin Ji <xji@...logixsemi.com>, devel@...verdev.osuosl.org,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        lkml <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Pi-Hsun Shih <pihsun@...omium.org>,
        Sheng Pan <span@...logixsemi.com>
Subject: Re: [PATCH v7 2/2] drm/bridge: anx7625: Add anx7625 MIPI DSI/DPI to
 DP bridge driver

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