linux-kernel - Re: [Intel-gfx] [PATCH v2] drm/i915: Try EDID bitbanging on HDMI after failed read

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Message-ID: <151492106622.21495.250228197580562391@mail.alporthouse.com>
Date:   Tue, 02 Jan 2018 19:24:26 +0000
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Stefan Brüns <stefan.bruens@...h-aachen.de>
Cc:     "David Airlie" <airlied@...ux.ie>, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH v2] drm/i915: Try EDID bitbanging on HDMI after
 failed read

Quoting Rodrigo Vivi (2018-01-02 19:12:18)
> On Sun, Dec 31, 2017 at 10:34:54PM +0000, Stefan Brüns wrote:
> > +     edid = drm_get_edid(connector, i2c);
> > +
> > +     if (!edid && !intel_gmbus_is_forced_bit(i2c)) {
> > +             DRM_DEBUG_KMS("HDMI GMBUS EDID read failed, retry using GPIO bit-banging\n");
> > +             intel_gmbus_force_bit(i2c, true);
> > +             edid = drm_get_edid(connector, i2c);
> > +             intel_gmbus_force_bit(i2c, false);
> > +     }
> 
> Approach seems fine for this case.
> I just wonder what would be the risks of forcing this bit and edid read when nothing is present on the other end?

Should be no more risky than using GMBUS as the bit-banging is the
underlying HW protocol; it should just be adding an extra delay to
the disconnected probe. Offset against the chance that it fixes
detection of borderline devices.

I would say that given the explanation above, the question is why not
apply it universally? (Bonus points for including the explanation as
comments.)
-Chris