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From: Doug Anderson <>
To: Sandeep Panda <>, Sean Paul <>
Cc: linux-arm-msm <>,
	Jeykumar Sankaran <>,, Andrzej Hajda <>,
	Archit Taneja <>,
	LKML <>,
	dri-devel <>,
	David Airlie <>,
	Laurent Pinchart <>
Subject: Re: [PATCH 2/2] drm/bridge: ti-sn65dsi86: Allow DT to set "HPD delay"
Date: Mon, 22 Oct 2018 13:54:26 -0700	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>


On Fri, Oct 19, 2018 at 1:20 PM Douglas Anderson <> wrote:
> Let's solve the mystery of commit bf1178c98930 ("drm/bridge:
> ti-sn65dsi86: Add mystery delay to enable()").  Specifically the
> reason we needed that mystery delay is that we weren't paying
> attention to HPD.
> Looking at the datasheet for the same panel that was tested for the
> original commit, I see there's a timing "t3" that times from power on
> to the aux channel being operational.  This time is specced as 0 - 200
> ms.  The datasheet says that the aux channel is operational at exactly
> the same time that HPD is asserted.
> Scoping the signals on this board showed that HPD was asserted 84 ms
> after power was asserted.  That very closely matches the magic 70 ms
> delay that we had.  ...and actually, in my esting the 70 ms wasn't
> quite enough of a delay and some percentage of the time the display
> didn't come up until I bumped it to 100 ms.
> To solve this, we tried to hook up the HPD signal in the bridge.
> ...but in doing so we found that that the bridge didn't report that
> HPD was asserted until ~280 ms after we powered it (!).  This is
> explained by looking at the sn65dsi86 datasheet section " HPD
> (Hot Plug/Unplug Detection)".  Reading there we see that the bridge
> isn't even intended to report HPD until 100 ms after it's asserted.
> ...but that would have left us at 184 ms.  The extra 100 ms
> (presumably) comes from this part in the datasheet:
> > The HPD state machine operates off an internal ring oscillator. The
> > ring oscillator frequency will vary [ ... ]. The min/max range in
> > the HPD State Diagram refers to the possible times based off
> > variation in the ring oscillator frequency.
> Given that the 280 ms we'll end up delaying if we hook up HPD is
> _slower_ than the 200 ms we could just hardcode, for now we'll solve
> the problem by just allowing boards to hardcode a value.  If someone
> using this part finds that they can get things to work more quickly by
> actually hooking up HPD that can always be a future patch.
> One last note is that I tried to solve this through another way: In
> ti_sn_bridge_enable() I tried to use various combinations of
> dp_dpcd_writeb() and dp_dpcd_readb() to detect when the aux channel
> was up.  In theory that would let me detect _exactly_ when I could
> continue and do link training.  Unfortunately even if I did an aux
> transfer w/out waiting I couldn't see any errors.  Possibly I could
> keep looping over link training until it came back with success, but
> that seemed a little overly hacky to me.
> Signed-off-by: Douglas Anderson <>
> ---
>  drivers/gpu/drm/bridge/ti-sn65dsi86.c | 45 ++++++++++++++++++++++-----
>  1 file changed, 37 insertions(+), 8 deletions(-)

Adding breadcrumbs to point to the new version of this patch at
<> AKA ("[4/6]
drm/bridge: ti-sn65dsi86: Remove the mystery delay").  I didn't call
that a v2 since it's a pretty different approach compared to this one,
but (assuming people are OK w/ it) it replaces this patch.



  reply	other threads:[~2018-10-22 20:54 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-19 20:19 [PATCH 1/2] dt-bindings: drm/bridge: sn65dsi86: Add panel-hpd-delay Douglas Anderson
2018-10-19 20:19 ` [PATCH 2/2] drm/bridge: ti-sn65dsi86: Allow DT to set "HPD delay" Douglas Anderson
2018-10-22 20:54   ` Doug Anderson [this message]
2018-10-24 18:32   ` spanda
2018-10-24 18:37     ` Doug Anderson
2018-10-21  9:06 ` [PATCH 1/2] dt-bindings: drm/bridge: sn65dsi86: Add panel-hpd-delay Laurent Pinchart
2018-10-22 20:52   ` Doug Anderson

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